*2.3.2 Sclereids*

*Plant Science - Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro*

surrounding the sieve tubes [8, 20, 23].

starch, which integrate with the companion cells [15].

sieve-tube-centric parenchyma appears (**Figure 4c**) and, as the name suggests, is

Although collectively described and referred to as axial phloem parenchyma, it is important to note that in many plants there will be distinct groups of phloem parenchyma within the phloem with quite different ergastic contents and therefore presumed different functions. Some of these specialized parenchyma cells may be considered secretory structures. Within a single plant, it is not uncommon that while some cells have crystals (especially when in contact with sclerenchyma), others have tannins, starch, and other substances. In apple trees (*Malus domestica*, *Rosaceae*) three types of axial parenchyma have been recorded: (1) crystal-bearing cells, (2) tannin- and starch-containing cells, and (3) those with no tannin or

Within bands of axial parenchyma, canals with a clear epithelium may be formed in many plant groups such as *Pinaceae*, *Anacardiaceae*, *Apiales*, a feature with strong phylogenetic signal. Some phloem parenchyma cells also act in the sustenance and support of the sieve elements, even when not derived from the same mother cell [7]. In longitudinal section, the axial phloem parenchyma may appear

While the phloem ages and moves away from the cambium, its structure dramatically change, and typically axial parenchyma cells enlarge (**Figures 4a** and **b**, **6c**), divide, and store more ergastic contents toward the nonconducting phloem. In plants with low fiber content, the dilatation undergone by the parenchyma cells typically provokes the collapse of the sieve elements. The axial parenchyma in the nonconducting phloem can dedifferentiate and give rise to new lateral meristems. In plants with multiple periderms, typically new phellogens are formed within the secondary phloem, compacting within the multiple periderms large quantities of dead, suberized phloem. In plants with variant secondary growth, especially lianas, new cambia might differentiate from axial phloem parenchyma cells [24]. In the Asian *Tetrastigma* (*Vitaceae*), new cambia were recorded differentiating from primary phloem paren-

Sclerenchymatic cells are those with thick secondary walls, commonly lignified. Sclerenchyma can be present or not in the phloem, and when present it typically gives structure to the tissue. For instance, a phloem with concentric layers of sclerenchyma cells is called stratified (**Figures 2e**, **3a**, and **4c**) [5]—not to be confused with storied, regarding the organization of the elements in tangential section. In Leguminosae, bands of phloem are associated to the concentric fiber bands

Older phloem shows more sclerification than younger phloem, and the sclerenchyma may also act as a barrier to bark attackers [21]. The sclerenchyma is typically divided in two categories: fibers and sclereids. These cell types differ mainly in form

Fibers are long and slender cells, derived from meristems, the fiber primordia [1, 26, 27]. In the primary phloem, fiber caps are sometimes found in association with the protophloem (**Figure 5a**) and are named protophloem fibers. Since only an ontogenetic study can evidence whether these fibers indeed differentiate within the protophloem, a term coined in the nineteenth century German and American literature, pericyclic fibers, has been recommended to be used instead of primary phloem

and size, but origin has also been used to distinguish them [26].

fusiform (not segmented) or in two up to several cells per strand [5].

**8**

chyma cells [25].

**2.3 Sclerenchyma**

(**Figure 4c**).

*2.3.1 Fibers*

Sclereids may have different forms and sizes (**Figure 6a**–**c**). Within the phloem, they are more typically square or polygonal (stone cells) and contain numerous pits and conspicuous pit canals. Holdheid [26] defines that a sclereid is a cell derived from the belated sclerification of a parenchyma cell, and that is in fact the rule in the majority of cases (**Figure 6a** and **b**). However, there are lineages in which the sclereids differentiate very close to the cambium (e.g., *Pleonotoma*, *Bignoniaceae*, **Figure 6c**; [20]), and it would be untrue to claim that the derivatives had a stage as a mature parenchyma cell [1]. In these cases, the form is enough to define the sclereid.

On the other hand, there are cases where long and slender cells derive from previously mature parenchyma cells and are morphologically difficult to distinguish from fibers. In these cases, these cells are called fiber sclereids and may be even in concentric layers, such as in apple trees and pears (*Malus domestica* and *Pyrus* 

#### **Figure 5.**

*Vascular fibers associated to eudicot and monocot primary structure. (a) Pericyclic fiber cap (fc) and primary phloem (pp) in Perianthomega vellozoi (Bignoniaceae). Secondary phloem (sp) beginning to be produced. Vascular bundles in monocotyledons. (b) Vascular bundle in the climber Calamus manan (Arecaceae) with fibers toward the phloem side. Phloem in two strands around a wide metaxylem vessel. (c) Vascular bundle of Vellozia alata (Velloziaceae), with fiber cap toward the xylem side. Phloem on the top side of the picture. (Picture credit to Marina Blanco Cattai). (d) Amphivasal bundle of Philodendron with fibers in the center of the vascular bundle and phloem surrounding it. Scalebars: a, b = 100 μm, c, d = 50 μm.*

#### **Figure 6.**

*Sclereids in the secondary phloem. (a) Sclereids (sc) differentiate from parenchyma cells (arrow) in the nonconducting phloem of Heteropterys intermedia (Malpighiaceae) TS, forming large clusters. (b) Longitudinal radial section of Heteropterys intermedia (Malpighiaceae) showing the sclereid masses. (c) In Pleonotoma tetraquetra (Bignoniaceae), the sclereids differentiate (arrow) close to the cambium within the conducting phloem. c, cambium; pe, periderm; sc, sclereid; sx, secondary xylem. Scalebars: a, b = 400 μm, c = 250 μm.*

*communis*, respectively; [15]). Sclereids can also develop with different arrangements in the phloem, being isolated and scattered or in clusters (**Figures 6a**–**c**) [5].

#### **2.4 Rays**

The rays in the conducting phloem have typically the same organization in terms of width, height, and cellular composition as the secondary xylem. In this respect the rays vary from uniseriate to multiseriate (**Figure 7a**) and may be homocellular or heterocellular (**Figure 7b**). Homocellular rays are those composed of cells of one shape, all procumbent or all upright (common in many shrubs). Heterocellular rays are those where more than one cell shape is present together (**Figure 7b**). Ray composition is appreciated in radial sections.

Because the vascular cambium produces much more xylem to the inside than phloem to the outside, phloem rays typically greatly dilate toward the periphery of the organ (**Figure 7c**). It is not uncommon that a dilatation meristem longitudinal to the cambium forms in some barks (**Figure 7c**), especially in families with very wide, wedge-like rays such as the *Malvaceae*. Plants with unicellular rays very rarely have dilatation by cell division [15, 26]. Instead, they have great lateral expansion of their single cells. Ray width can be only determined in tangential sections.

Rays are typically exclusively parenchymatic; however, in many species sieve elements appear in the rays and are called ray sieve cells or radial sieve cells [5, 28, 29]. These cells were recorded connecting two different sieve tubes (collections of sieve tube elements). Ray sieve elements seem to be present in taxa where perforated ray cells have been also recorded [30].

#### **3. Structure and development of primary and secondary phloem**

#### **3.1 Primary phloem**

The primary phloem derives from the embryo in the seed and the procambium from the organ's apices. Similarly to the primary xylem, the primary phloem is

**11**

**Figure 7.**

*Phloem: Cell Types, Structure, and Commercial Uses DOI: http://dx.doi.org/10.5772/intechopen.88162*

*tube element. Scalebars: a, b = 100 μm, c = 300 μm.*

divided in protophloem and metaphloem (**Figure 1d**), with the protophloem differentiating first, while the plant is still elongating, and the metaphloem differentiating last. The phloem is always exarch, independently of the organ. Protophloem sieve elements sometimes lack companion cells, such as in *Arabidopsis*, and in this case the sieve elements are sustained by other neighboring parenchyma cells. Commonly, the protophloem quickly becomes obliterated and loses function. In plants without secondary growth, the metaphloem will be conducted during the entire life of the plant, as in the monocotyledons (**Figure 5b**–**d**) [11]. Different vascular plant lineages display different arrangements of the primary xylem and phloem, depending on the stele type. Two main types of steles exist, the protostele and the siphonostele. In the protostele, the entire center of the organ is composed of vascular tissue (**Figure 1a**), with the phloem in strands alternated with a central xylem in the protostele, haplostele, and actinostele (**Figure 1a**), while primary phloem is interspersed in the protostele plectostele [6]. The roots of all the vascular plants are protostelic (**Figure 1a**). The stems, however, can vary. In the lycophytes, they are always protostelic, while in the ferns (monilophytes) they might be protostelic, such as in *Psilotum*, or in all other range of siphonostelic steles [31]. The siphonostele evolved in concert with the macrophytes and resulted in the formation of a central pith derived from the ground meristem. No lineage displays as much diversity in the primary vasculature architecture as do the ferns. In the seed plants, that is, gymnosperms and angiosperms, the stem stele is always a syphonostele, either a eustele, where discrete vascular bundles form a concentric ring, or the atactostele, a type of stele exclusive of the monocotyledons where the bundles are scattered in the entire stem center. Some lineages of eudicotyledons and *Magnoliids* have evolved another subtype of siphonostele, the polycyclic eustele, where more than one ring of bundles is present, such as in *Piperaceae* and *Nyctaginaceae*. The primary phloem is simpler than the secondary phloem and is basically formed by sieve elements and parenchyma cells (**Figure 1a**–**d**). Fiber caps are commonly present, and they might be phloematic (**Figure 5a**). For a discussion on their origin, check the section on fibers above. The position of the phloem is typically external or abaxial to the xylem, but in some lineages the bundles are bicollateral

*Rays in the secondary phloem. (a) Longitudinal tangential section of Brachylaena transvaalensis (Asteraceae) showing storied structure, biseriate to triseriate rays (r), sieve tube elements with simple sieve plate (s) and axial parenchyma cells composed of 4–5 cells, and fibers. (b) Longitudinal radial section of Brachylaena transvaalensis (Asteraceae) showing heterocellular rays (r), with body procumbent and one row of marginal square cells. Fibers (f) in bands. (c) Ray dilatation (rd) by the formation of a dilatation meristem in the center of the ray in Perianthomega vellozoi (Bignoniaceae). f, fiber; p, axial parenchyma cell; r, ray; s, sieve* 

#### **Figure 7.**

*Plant Science - Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro*

*communis*, respectively; [15]). Sclereids can also develop with different arrangements in the phloem, being isolated and scattered or in clusters (**Figures 6a**–**c**) [5].

*Sclereids in the secondary phloem. (a) Sclereids (sc) differentiate from parenchyma cells (arrow) in the nonconducting phloem of Heteropterys intermedia (Malpighiaceae) TS, forming large clusters.* 

*(b) Longitudinal radial section of Heteropterys intermedia (Malpighiaceae) showing the sclereid masses. (c) In Pleonotoma tetraquetra (Bignoniaceae), the sclereids differentiate (arrow) close to the cambium within the conducting phloem. c, cambium; pe, periderm; sc, sclereid; sx, secondary xylem. Scalebars: a, b = 400 μm,* 

The rays in the conducting phloem have typically the same organization in terms of width, height, and cellular composition as the secondary xylem. In this respect the rays vary from uniseriate to multiseriate (**Figure 7a**) and may be homocellular or heterocellular (**Figure 7b**). Homocellular rays are those composed of cells of one shape, all procumbent or all upright (common in many shrubs). Heterocellular rays are those where more than one cell shape is present together (**Figure 7b**). Ray

Because the vascular cambium produces much more xylem to the inside than phloem to the outside, phloem rays typically greatly dilate toward the periphery of the organ (**Figure 7c**). It is not uncommon that a dilatation meristem longitudinal to the cambium forms in some barks (**Figure 7c**), especially in families with very wide, wedge-like rays such as the *Malvaceae*. Plants with unicellular rays very rarely have dilatation by cell division [15, 26]. Instead, they have great lateral expansion of their

Rays are typically exclusively parenchymatic; however, in many species sieve elements appear in the rays and are called ray sieve cells or radial sieve cells [5, 28, 29]. These cells were recorded connecting two different sieve tubes (collections of sieve tube elements). Ray sieve elements seem to be present in taxa where perforated ray

The primary phloem derives from the embryo in the seed and the procambium from the organ's apices. Similarly to the primary xylem, the primary phloem is

single cells. Ray width can be only determined in tangential sections.

**3. Structure and development of primary and secondary phloem**

**10**

**2.4 Rays**

*c = 250 μm.*

**Figure 6.**

composition is appreciated in radial sections.

cells have been also recorded [30].

**3.1 Primary phloem**

*Rays in the secondary phloem. (a) Longitudinal tangential section of Brachylaena transvaalensis (Asteraceae) showing storied structure, biseriate to triseriate rays (r), sieve tube elements with simple sieve plate (s) and axial parenchyma cells composed of 4–5 cells, and fibers. (b) Longitudinal radial section of Brachylaena transvaalensis (Asteraceae) showing heterocellular rays (r), with body procumbent and one row of marginal square cells. Fibers (f) in bands. (c) Ray dilatation (rd) by the formation of a dilatation meristem in the center of the ray in Perianthomega vellozoi (Bignoniaceae). f, fiber; p, axial parenchyma cell; r, ray; s, sieve tube element. Scalebars: a, b = 100 μm, c = 300 μm.*

divided in protophloem and metaphloem (**Figure 1d**), with the protophloem differentiating first, while the plant is still elongating, and the metaphloem differentiating last. The phloem is always exarch, independently of the organ. Protophloem sieve elements sometimes lack companion cells, such as in *Arabidopsis*, and in this case the sieve elements are sustained by other neighboring parenchyma cells. Commonly, the protophloem quickly becomes obliterated and loses function. In plants without secondary growth, the metaphloem will be conducted during the entire life of the plant, as in the monocotyledons (**Figure 5b**–**d**) [11]. Different vascular plant lineages display different arrangements of the primary xylem and phloem, depending on the stele type. Two main types of steles exist, the protostele and the siphonostele. In the protostele, the entire center of the organ is composed of vascular tissue (**Figure 1a**), with the phloem in strands alternated with a central xylem in the protostele, haplostele, and actinostele (**Figure 1a**), while primary phloem is interspersed in the protostele plectostele [6]. The roots of all the vascular plants are protostelic (**Figure 1a**). The stems, however, can vary. In the lycophytes, they are always protostelic, while in the ferns (monilophytes) they might be protostelic, such as in *Psilotum*, or in all other range of siphonostelic steles [31]. The siphonostele evolved in concert with the macrophytes and resulted in the formation of a central pith derived from the ground meristem. No lineage displays as much diversity in the primary vasculature architecture as do the ferns. In the seed plants, that is, gymnosperms and angiosperms, the stem stele is always a syphonostele, either a eustele, where discrete vascular bundles form a concentric ring, or the atactostele, a type of stele exclusive of the monocotyledons where the bundles are scattered in the entire stem center. Some lineages of eudicotyledons and *Magnoliids* have evolved another subtype of siphonostele, the polycyclic eustele, where more than one ring of bundles is present, such as in *Piperaceae* and *Nyctaginaceae*.

The primary phloem is simpler than the secondary phloem and is basically formed by sieve elements and parenchyma cells (**Figure 1a**–**d**). Fiber caps are commonly present, and they might be phloematic (**Figure 5a**). For a discussion on their origin, check the section on fibers above. The position of the phloem is typically external or abaxial to the xylem, but in some lineages the bundles are bicollateral

(**Figure 1b**), and phloem is present both inside and outside (abaxial and adaxial), while in amphivasal bundles, the xylem encircles the phloem (**Figure 5d**), as in the secondary vascular tissues of some *Asparagales* [32, 33] and *Iridaceae* corms [34]. In some plant families and orders, intraxylary phloem (perimedullar phloem islands) is a synapomorphy, such as in the order *Myrtales* and in the families *Apocynaceae* and *Convolvulaceae* [35]. These phloem strands are initially primary, but a cambium can differentiate between the protoxylem and the phloem strands and develop secondary tissues inside of the pith.

#### **3.2 Secondary phloem**

Being derived from the cambium, the secondary phloem will share a number of characteristics with the secondary xylem. For instance, it is divided in an axial and radial system. The axial system is composed of sieve elements, axial parenchyma cells, and fibers, and the radial system is formed by rays, which are typically parenchymatic (**Figure 2a**–**c**). Similar to secondary xylem, the secondary phloem can be storied (**Figure 7a**) or non-storied (**Figure 2b** and **c**), depending whether the cambial mother cells are organized in tiers or not.

Some trees will have growth rings, with an early and a late phloem, both in temperate and tropical regions, but their characterization is only possible with periodical collections [5]. Sometimes, but not always, the fiber band width gives a hint on the presence of growth rings or the formation of very small sieve elements in the late phloem [1, 5].

#### *3.2.1 Secondary phloem of gymnosperms*

In conifers (except Gnetales) the secondary phloem is typically marked by an alternation of axial cell types (**Figure 3a** and **b**), uniseriate rays, and, in many lineages, axial and radial resin canals (e.g., *Pinaceae* and *Cupressaceae*). In the *Pinaceae*, the phloem is marked by the presence of an alternation of sieve cells and bands of axial parenchyma with phenolic contents, some also with druses. In the nonconducting phloem of *Pinaceae*, sclereids differentiate. In all other conifers, in addition to the alternation of parenchyma bands and sieve cells, fiber bands are present (**Figure 3a** and **b**). Therefore, sieve cells, parenchyma cells with phenolic content, and bands of fibers appear in alternation in non-*Pinaceae* and Gnetales conifers, including *Araucariaceae*, *Cupressaceae*, *Podocarpaceae*, *Taxaceae*, and *Taxodiaceae* [8, 21]. Another marked difference of these conifers compared to *Pinaceae* is that they contain a lot of crystals in their cell walls, including in Gnetales (see New World *Ephedra*; [36]), while in *Pinaceae* they are exclusively inside of idioblastic cells.

In other gymnosperms, in particular in Gnetales and Cycads, the first remarkable difference is the presence of very wide, multiseriate rays alternating with uniseriate rays. The wide rays in both groups have, however, evolved independently, since Cycads are a sister to all other gymnosperms, while Gnetales are within the conifers, as sister to the Pinaceae [31, 37]. In *Cyca* and the extinct *Cycadoidea*, sieve cells and phloem parenchyma alternate with fibers, which can be in tangential bands or not [38, 39]. In *Cyca*, the sieve cells appear in radial rolls [38], while in *Cycadoidea* there is a constant alternation of one sieve cell or phloem parenchyma to one fiber [39]. The nonconducting phloem of *Cycas* is marked by the collapse of sieve cells, enlargement of the axial parenchyma cells, ray dilatation, and sclerosis of some parenchymatic cells [38]. More than one ring of secondary phloem is present in some Cycads (e.g., *Cycas, Encephalartos*, *Lepidozamia*, and *Macrozamia*) and Gnetales (e.g., *Gnetum*), given that they have successive cambia [38, 40].

**13**

*Phloem: Cell Types, Structure, and Commercial Uses DOI: http://dx.doi.org/10.5772/intechopen.88162*

*3.2.2 Secondary phloem of angiosperms*

especially when associated with fibers.

and this is a constant character among them.

**4. Phloem activity**

only around the sieve tube elements (**Figure 4d**) [5].

Within the Gnetales, in *Ephedra* axial parenchyma cells are interspersed with sieve cells (**Figure 4a**), and fiber may or may not be present and are typically gelatinous [36]. Fiber sclereids and/or sclereids appear in the nonconducting phloem of other species [13, 22]. In the nonconducting phloem of *Ephedra*, the sieve cells and Strasburger cells collapse with the enlargement of the axial and radial parenchyma cells (**Figure 4a**) with more ergastic contents [13]. In *Gnetum*, large areas of parenchyma sclerify, forming bands in the nonconducting phloem. The secondary phloem of *Welwitschia* is described as containing a large amount of fibers [21].

Within the angiosperms, the diversity of phloem cell type arrangements reaches its maximum. The structure can be storied (**Figure 7a**) or non-storied (**Figure 2b** and **c**); sclerenchyma can be present or lacking. The rays may be uni-, bi-, or multiseriate. A large array of secretory cells may be encountered, such as resin canals, laticifers, and mucilaginous cells. Crystalliferous parenchyma is also very common,

The variation in cell type arrangements can be of taxonomic interest. Sieve elements can vary in morphology and arrangement. They can be solitary (**Figure 2f**), scattered in the phloem (e.g., *Eucalyptus*, *Myrtaceae*), in clusters (e.g., *Malvaceae*; **Figures 2a**, **d** and **4c**), and in radial or tangential rows (many *Bignoniaceae*; [20]; **Figure4d**). The functional significance of the different arrangements is unknown to date, although this

The presence, type, and arrangements of fibers and sclereids are one of the most

informative characters in the bark [4]. In *Apocynaceae*, the fibers are completely absent, except in *Aspidosperma*, the sister group of all other *Apocynaceae* [35]. In *Aspidosperma*, they can appear solitary scattered across the phloem or in clusters. In some lineages, fibers appear in concentric alternating bands, as in *Leguminosae* (*Papilionoideae)*, *Mimosoideae* (**Figure 4c**) [41], *Bignoniaceae* [20], and *Malvaceae*,

Phloem parenchyma more commonly constitute the background tissue in the phloem but can also be distributed in bands (**Figure 4b** and **c**), radial rows, or even

The classic theory of phloem transport is that proposed by Ernst Münch [42], and it involves the formation of an osmotic pressure transport gradient, where certain zones act as sources of sugars (leaves and storage organs), while others act as sinks. Experiments showed that the concentration gradients were always seen to be positive in the direction of flow [43], supporting Münch's postulate. In a system where transport goes against the direction of transpiration, its functionality relies on the presence of a plasma membrane across the entire system to create an osmotic pressure, hence the need of a conducting system with living cells [44]. Recent studies have been refining aspects involved in the photosynthate conduction to explain long-distance transports across large trees with such a simple system [44, 45]. A direct role of intracellular calcium has also been reported in the dissolution of nondispersive P-proteins and facilitation of transport [46]. Likely, the anatomical structure of the phloem discussed in the previous sections of this chapter will prove to play a role in the system. For instance, phloem sieve element length scale with the tree sizes and sieve plate type [45]. It was also shown that sieve element's diameter, length, and pore width increase from the top to the base of the trees [47, 48].

is one of the features in the phloem with the strongest phylogenetic signal.

#### *Phloem: Cell Types, Structure, and Commercial Uses DOI: http://dx.doi.org/10.5772/intechopen.88162*

*Plant Science - Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro*

secondary tissues inside of the pith.

the cambial mother cells are organized in tiers or not.

**3.2 Secondary phloem**

in the late phloem [1, 5].

*3.2.1 Secondary phloem of gymnosperms*

(**Figure 1b**), and phloem is present both inside and outside (abaxial and adaxial), while in amphivasal bundles, the xylem encircles the phloem (**Figure 5d**), as in the secondary vascular tissues of some *Asparagales* [32, 33] and *Iridaceae* corms [34]. In some plant families and orders, intraxylary phloem (perimedullar phloem islands) is a synapomorphy, such as in the order *Myrtales* and in the families *Apocynaceae* and *Convolvulaceae* [35]. These phloem strands are initially primary, but a cambium can differentiate between the protoxylem and the phloem strands and develop

Being derived from the cambium, the secondary phloem will share a number of characteristics with the secondary xylem. For instance, it is divided in an axial and radial system. The axial system is composed of sieve elements, axial parenchyma cells, and fibers, and the radial system is formed by rays, which are typically parenchymatic (**Figure 2a**–**c**). Similar to secondary xylem, the secondary phloem can be storied (**Figure 7a**) or non-storied (**Figure 2b** and **c**), depending whether

Some trees will have growth rings, with an early and a late phloem, both in temperate and tropical regions, but their characterization is only possible with periodical collections [5]. Sometimes, but not always, the fiber band width gives a hint on the presence of growth rings or the formation of very small sieve elements

In conifers (except Gnetales) the secondary phloem is typically marked by an alternation of axial cell types (**Figure 3a** and **b**), uniseriate rays, and, in many lineages, axial and radial resin canals (e.g., *Pinaceae* and *Cupressaceae*). In the *Pinaceae*, the phloem is marked by the presence of an alternation of sieve cells and bands of axial parenchyma with phenolic contents, some also with druses. In the nonconducting phloem of *Pinaceae*, sclereids differentiate. In all other conifers, in addition to the alternation of parenchyma bands and sieve cells, fiber bands are present (**Figure 3a** and **b**). Therefore, sieve cells, parenchyma cells with phenolic content, and bands of fibers appear in alternation in non-*Pinaceae* and Gnetales conifers, including *Araucariaceae*, *Cupressaceae*, *Podocarpaceae*, *Taxaceae*, and *Taxodiaceae* [8, 21].

Another marked difference of these conifers compared to *Pinaceae* is that they contain a lot of crystals in their cell walls, including in Gnetales (see New World *Ephedra*;

In other gymnosperms, in particular in Gnetales and Cycads, the first remarkable difference is the presence of very wide, multiseriate rays alternating with uniseriate rays. The wide rays in both groups have, however, evolved independently, since Cycads are a sister to all other gymnosperms, while Gnetales are within the conifers, as sister to the Pinaceae [31, 37]. In *Cyca* and the extinct *Cycadoidea*, sieve cells and phloem parenchyma alternate with fibers, which can be in tangential bands or not [38, 39]. In *Cyca*, the sieve cells appear in radial rolls [38], while in *Cycadoidea* there is a constant alternation of one sieve cell or phloem parenchyma to one fiber [39]. The nonconducting phloem of *Cycas* is marked by the collapse of sieve cells, enlargement of the axial parenchyma cells, ray dilatation, and sclerosis of some parenchymatic cells [38]. More than one ring of secondary phloem is present in some Cycads (e.g., *Cycas, Encephalartos*, *Lepidozamia*, and *Macrozamia*) and Gnetales (e.g., *Gnetum*), given that they have successive

[36]), while in *Pinaceae* they are exclusively inside of idioblastic cells.

**12**

cambia [38, 40].

Within the Gnetales, in *Ephedra* axial parenchyma cells are interspersed with sieve cells (**Figure 4a**), and fiber may or may not be present and are typically gelatinous [36]. Fiber sclereids and/or sclereids appear in the nonconducting phloem of other species [13, 22]. In the nonconducting phloem of *Ephedra*, the sieve cells and Strasburger cells collapse with the enlargement of the axial and radial parenchyma cells (**Figure 4a**) with more ergastic contents [13]. In *Gnetum*, large areas of parenchyma sclerify, forming bands in the nonconducting phloem. The secondary phloem of *Welwitschia* is described as containing a large amount of fibers [21].

#### *3.2.2 Secondary phloem of angiosperms*

Within the angiosperms, the diversity of phloem cell type arrangements reaches its maximum. The structure can be storied (**Figure 7a**) or non-storied (**Figure 2b** and **c**); sclerenchyma can be present or lacking. The rays may be uni-, bi-, or multiseriate. A large array of secretory cells may be encountered, such as resin canals, laticifers, and mucilaginous cells. Crystalliferous parenchyma is also very common, especially when associated with fibers.

The variation in cell type arrangements can be of taxonomic interest. Sieve elements can vary in morphology and arrangement. They can be solitary (**Figure 2f**), scattered in the phloem (e.g., *Eucalyptus*, *Myrtaceae*), in clusters (e.g., *Malvaceae*; **Figures 2a**, **d** and **4c**), and in radial or tangential rows (many *Bignoniaceae*; [20]; **Figure4d**). The functional significance of the different arrangements is unknown to date, although this is one of the features in the phloem with the strongest phylogenetic signal.

The presence, type, and arrangements of fibers and sclereids are one of the most informative characters in the bark [4]. In *Apocynaceae*, the fibers are completely absent, except in *Aspidosperma*, the sister group of all other *Apocynaceae* [35]. In *Aspidosperma*, they can appear solitary scattered across the phloem or in clusters. In some lineages, fibers appear in concentric alternating bands, as in *Leguminosae* (*Papilionoideae)*, *Mimosoideae* (**Figure 4c**) [41], *Bignoniaceae* [20], and *Malvaceae*, and this is a constant character among them.

Phloem parenchyma more commonly constitute the background tissue in the phloem but can also be distributed in bands (**Figure 4b** and **c**), radial rows, or even only around the sieve tube elements (**Figure 4d**) [5].
