**4. Phloem activity**

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].

Across the entire pathway, sugars are removed from the system to sustain all cells in the plant body. This mechanism is only possible with the concerted mechanism between sieve elements and their close related cells (Strasburger cells and companion cells), with these accompanying cells constantly channeling substances and macromolecules toward the sieve elements [44]. The Strasburger and companion cells carry the loading and unloading of the sieve elements. Given the function of loading and unloading, the companion cell-sieve tube element size ratio is directly related to being in the source or the sink of sugars [44]. For instance, in leaves the companion cells are typically much larger, for they have the high demand of constantly loading the sieve tubes. In areas of release of the sugars (unloading), the companion cells are much smaller or even absent [44].

### **5. Economic uses**

In the economic uses, it is not always easy to distinguish the use of the phloem from that of the periderm, since both together compose the bark of a woody plant. The phloem corresponds to the inner bark, and the periderm to the outer bark. The bark has a long history of utilization, from the production of remedies [49], aphrodisiacs (yohimbe), insecticides [50], dyes, tannins [50], angostura, fibers [51], gums and resins [50], latex, and flavorings [52].

In indigenous groups from British Columbia (Canada) and Tanzania, barks from dozens of species of woody plants are used as carbohydrate food, medicine, fibers, and structural material [50, 53]. In Mexico the bark of *Ficus* is used since prehispanic times to create a type of paper called *papel amate* (from the náhuatl paper = ámatl), used, for example, to create the Aztec codices.

The rubber tree, *Hevea brasiliensis* (*Euphorbiaceae*), is known from the extraction of latex to the production of rubber. Laticifers are present in concentric rings in the secondary phloem of the rubber tree and are an important economic asset in some tropical countries. Bark residues have also been considered for mulching [53–55], to build particle boards [56, 57], as fuel, and a source of food for ruminants [52].

### **Acknowledgements**

I would like to express gratitude to Ray F. Evert, Veronica Angyalossy, Carmen Marcati, and André C. Lima for allowing their slide collections to be photographed and Leyde N. Nunes for the photo of *Tetrapterys* leaf, Solange Mazzoni Viveiros for photos of *Cucurbita*, and Marina Blanco Cattai for picture of *Vellozia*.

**15**

**Author details**

Marcelo R. Pace

de México, Mexico City, Mexico

provided the original work is properly cited.

Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: marcelo.pace@ib.unam.mx

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

### **Conflict of interest**

The authors declare no conflict of interest.

*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*

companion cells are much smaller or even absent [44].

[51], gums and resins [50], latex, and flavorings [52].

paper = ámatl), used, for example, to create the Aztec codices.

**5. Economic uses**

**Acknowledgements**

**Conflict of interest**

Across the entire pathway, sugars are removed from the system to sustain all cells in the plant body. This mechanism is only possible with the concerted mechanism between sieve elements and their close related cells (Strasburger cells and companion cells), with these accompanying cells constantly channeling substances and macromolecules toward the sieve elements [44]. The Strasburger and companion cells carry the loading and unloading of the sieve elements. Given the function of loading and unloading, the companion cell-sieve tube element size ratio is directly related to being in the source or the sink of sugars [44]. For instance, in leaves the companion cells are typically much larger, for they have the high demand of constantly loading the sieve tubes. In areas of release of the sugars (unloading), the

In the economic uses, it is not always easy to distinguish the use of the phloem from that of the periderm, since both together compose the bark of a woody plant. The phloem corresponds to the inner bark, and the periderm to the outer bark. The bark has a long history of utilization, from the production of remedies [49], aphrodisiacs (yohimbe), insecticides [50], dyes, tannins [50], angostura, fibers

In indigenous groups from British Columbia (Canada) and Tanzania, barks from dozens of species of woody plants are used as carbohydrate food, medicine, fibers, and structural material [50, 53]. In Mexico the bark of *Ficus* is used since prehispanic times to create a type of paper called *papel amate* (from the náhuatl

The rubber tree, *Hevea brasiliensis* (*Euphorbiaceae*), is known from the extraction of latex to the production of rubber. Laticifers are present in concentric rings in the secondary phloem of the rubber tree and are an important economic asset in some tropical countries. Bark residues have also been considered for mulching [53–55], to build particle boards [56, 57], as fuel, and a source of food for ruminants [52].

I would like to express gratitude to Ray F. Evert, Veronica Angyalossy, Carmen Marcati, and André C. Lima for allowing their slide collections to be photographed and Leyde N. Nunes for the photo of *Tetrapterys* leaf, Solange Mazzoni Viveiros for

photos of *Cucurbita*, and Marina Blanco Cattai for picture of *Vellozia*.

The authors declare no conflict of interest.

**14**
