222 Pla?it Biology 



II. THE STEM 



The development and differentiation of the tissues of a stem occur 

 in the buds at the ends of the stem and its branches. The growing end 

 of a stem does not have a root cap as does a root, but it does have an 

 embryonic region, an elongation region, a maturation region, and a 

 mature region. The many similar cells of the embryonic region of the 

 bud divide rapidly by mitosis. Growth and cell differentiation occur in 

 the bud so that the youngest part of the stem or branch is nearest the 

 bud. The elongation region develops after the bud opens. The differ- 

 entiation of the cells of the stem to form xylem, phloem, parenchyma, 

 etc., is generally similar to the corresponding cells of the root. 



Study of a Stem of a Dicotyledonous Plant. — If a thin section of 

 such a plant as the sunflower is studied, the following structures are 

 visible (Fig. 57): (1) epidermis, (2) cortex, and (3) stele. The epi- 

 dermis is composed of a single layer of flattened cells for protection. The 

 walls are infiltrated with a waxy substance (cutin) to prevent loss of 

 water. The cortex consists of parenchyma cells and mechanical tissues. 

 The parenchyma cells of the cortex are continuous with the large pa- 

 renchyma cells of the pith. These two groups of parenchyma cells sep- 

 arate adjacent vascular bundles. The mechanical tissue is formed of 

 elongated, thick-walled cells to give rigidity to the stem. The tissues of 

 the stele of the stem are quite different from the stele of the root. The 

 conducting tissues of the stem stele consist of a series of vascular bundles 

 arranged in the form of a ring, leaving a large central area of pith. Each 

 vascular bundle is composed of xylem and phloem separated by cambium. 

 In young stems the large parenchyma cells of the pith and cortex are con- 

 tinuous between the vascular bundles. The parenchyma forms radiating 

 medullary rays (pith rays) between the vascular bundles. In each vascu- 

 lar bundle the phloem lies external to the cambium and the xylem in- 

 ternal to it. The xylem cells have thick walls as in the root xylem and 

 conduct liquids from the roots to the leaves, etc. The annular rings are 

 even more noticeable in stems than they are in roots, although they are 

 formed in much the same manner in each. The phloem conducts ma- 

 terials downward toward the roots. 



Study of a Stem of a Monocotyledonous Plant. — If a thin section of 

 such a plant as corn is studied, certain differences will be noted (Fig. 

 60). In the monocotyledonous stems the vascular bundles are scattered 

 throughout the parenchyma. Each vascular bundle contains only pri- 

 mary xylem and phloem but no cambium separating them. It is impos- 



