Preface

This book presents comprehensive information on plant anatomy, physiology, and morphogenesis as well as the conditions of the different environments to which plants are exposed. It highlights the importance of knowledge of the anatomy of plant tissues for different applications. In addition to the variety of physiological studies presented here, the book also emphasizes anatomical studies in botanical quality control of medicinal herbs with human health benefits.

Chapter 1, by Marcelo Pace (Mexico), describes in detail the origin and structure of phloem and its cell types. The author, in addition to analyzing the composition of phloem in different groups of plants, highlights some of the known commercial uses of this tissue.

Several contributions deal with the relationship between various hormones with processes in plants, both ex vitro and in vitro, and under different conditions of stress. The chapter by Dongyang Xu (China) and Masaaki Watahiki (Japan) introduces the development of root systems regulated by various phytohormones like auxin, cytokinin, and others. In their chapter, Hsiang-Ting and Wen-Lii, Huang (Taiwan) study the relationship between endogenous phytohormone signals and carbohydrate metabolism during regenerable callus induction induced by osmotic stress in rice. Based on the prolific role of jasmonates and their derivatives in different fields of biological sciences, Shivani Lalotra, A. Hemantaranjan, B. R. Yashu, R. Srivastava, and S. Kumar (India), in their chapter, discuss these phytohormones in terms of future agricultural, biotechnological, and physiological research. The onset of leaf senescence is a highly regulated developmental program that is controlled by both genetics and the environment. Otto Teixeira Fraga, B. Paes de Melo, L. Fernando de Camargos, D. Pellanda Fagundes, C. Cabral Oliveira, E. Bassi Simoni, Pedro A. Braga dos Reis, and E- Pacheco Batista Fontes (Brazil) analyze a regulatory circuit integrating stress-induced with natural leaf senescence.

Hafsi M. and Guendouz A. (Algeria) evaluate the performance of some durum wheat genotypes and test the efficiency of using senescence parameters in screening under semi-arid conditions.

Admasu Moges and Yohannes Moges (Ethiopia) present a review that documents medicinal plants used for traditional treatments as well as their parts, usage, ecology, and quality control. Accordingly, they review eighty medicinal plant species whose leaves and roots are the main parts used for preparation of traditional medicines. Chromatography, electrophoretic, macroscopic, and microscopic techniques, and pharmaceutical practice, are mainly used for quality control of herbal medicines.

> **Ana Maria Gonzalez** Academic Editor, Instituto de Botánica del Nordeste. UNNE-CONICET, Argentina

> > **Maria Vitoria Rodriguez** Universidad Nacional de Rosario, Argentina

> > > **1**

**Chapter 1**

**Abstract**

*Marcelo R. Pace*

mercial uses of this tissue.

**1. Introduction**

(**Figure 1b**).

Phloem: Cell Types, Structure,

Phloem is the vascular tissue in charge of transport and distribution of the organic nutrients. The phloem is also a pathway to signaling molecules and has a structural function in the plant body. It is typically composed of three cell types: sieve elements, parenchyma, and sclerenchyma. The sieve elements have the main function of transport and typically have lost their nuclei and other organelles in the course of their specialization. Hence, the sieve elements rely on specialized neighboring parenchyma cells to sustain all of their physiological function and activities. All cell types of the phloem may vary morphologically and in their distribution in the tissue, and this diversity is taxonomic and functionally informative. The phloem can be of primary or secondary origin, being derived from either procambium or cambium, respectively. Some vascular plant lineages have exclusive primary phloem, such as the lycophytes, ferns, and the monocotyledons, and the sieve elements will be long living in these taxa. In plants with secondary growth, the secondary phloem is formed, and typically the primary phloem collapses. Because new secondary phloem is constantly formed, the longevity of sieve elements in the secondary plant body is much more reduced. In this chapter, the structure of the phloem and its cell types are described in detail and also some of the known com-

**Keywords:** sieve tube, sieve tube element, companion cells, bark

Phloem is the vascular plant tissue responsible for the transport and distribution of sugars produced by the photosynthesis. Since the plant is a continuum, phloem will be found in the external part of root cylinders (**Figure 1a**), in the stem vascular bundles (**Figure 1b**) and in the abaxial part of the venations of every single leaf (**Figure 1c**). While the most common is to have the phloem external to the xylem in roots and stems and abaxial in leaves, some exceptions exist and are usually taxon specific. The phloem found in the inside is named internal or intraxylary phloem

As a constitutive tissue in the plant body, phloem functions extrapolate its main function of sugar transport, including transport of signalizing molecules such as mRNAs, hormones, defenses from biotic and abiotic agents, sustenance of the organs, gas exchange, and storage of many ergastic materials, such as starch, calcium oxalate crystals, and tannins. Parenchymatic cells of the phloem can also give rise to new meristems, such as the phellogen or cork cambium. All vascular plants have phloem, which typically includes specialized living conducting cells

and Commercial Uses

**Nihal Gören Sağlam** Istanbul University Department of Botany, Turkey
