248 - Multicellular Plants 



called vascular bundles. The vascular bun- 

 dles of the stem extend downward into the 

 root and upward into the leaves, forming a 

 continuous system of channels throughout 

 the plant. In addition to vessels and sieve 

 tubes, most vascular bundles contain consid- 

 erable strengthening tissue, sclerenchyma 

 tissue. However, much of the strength and 

 flexibility of the stem and its branches is due 

 to the mechanical properties of the vascular 

 bundles (Fig. 13-13). 



Microscopic Structure of a Stem. The tissues 

 of the stem are organized around the vascular 

 bundles, although the arrangement of the 

 bundles differs considerably in different 

 plants. Main attention will be given to the 

 stem of a clover plant, which is quite typical 

 of the young stems of dicotyledonous plants 

 generally. 



In cross section, the clover stem displays a 

 circular arrangement of the vascular bundles 

 (Fig. 13-13). Accordingly, it is possible to sub- 

 divide the stem into three concentric regions: 

 (1) the pith, a central core of colorless paren- 

 chyma; (2) the vascular region, comprised by 

 the ring of bundles, taken collectively; and 

 (3) the cortex, the tissues that surround the 

 ring of bundles. 



Figure 13-13 shows that each vascular bun- 

 dle is subdivided into an inner part, the 

 xylem, and an outer part, the phloem, by the 

 cambium, a layer of meristem tissue. All 

 growth in the girth of the stem results from 

 the activity of the cambium cells, which keep 

 multiplying and differentiating to form the 

 other tissues. New phloem — which consists 

 mainly ol sieve tubes — is formed along the 

 outer margin of the cambium; and new 

 xylem — mainly ducts — is formed by the cam- 

 bium along its inner margin. 



Surrounding the vascular region lie the 

 tissues ol the cortex, which commonly is 

 called the bark; and between the bundles lie 

 the rays, the strands of tissue that extend 

 radially from the pith to the cortex (Fig. 13- 

 8). The cortex, together with its superficial 

 layer of epidermis, serves chiefly as a protec- 

 tive cover, although in young stems the cor- 



tex may contain chlorenchyma and perform 

 a limited amount of photosynthesis. The 

 pith consists of colorless parenchyma, in 

 which the large thin-walled cells serve as 

 storage centers, chiefly of starch grains. In 

 older stems, which, unlike the clover stem, 

 continue to grow from year to year, the cells 

 of the rays become elongate in a horizontal 

 direction. This growth indicates that the rays 

 act as channels for the radial distribution of 

 substances, from the vascular tissues outward 

 into the cortex and inward into the pith. 



During the first year of growth, woody 

 stems such as are characteristic of most trees 

 and shrubs closely resemble the clover stem. 

 But in woody sterns, the cambium gives rise 

 each year to an additional layer of xylem 

 on the inside, and another (much thinner) 

 layer of phloem on the outside. These yearly 

 deposits of xylem in woody stems are clearly 

 differentiated as the annual rings (Fig. 13-14). 

 The annual rings are distinguishable because 

 the ducts that are formed in the spring of 

 each year are larger than those that are 

 formed in summer; and because fewer wood 

 fibers (p. 213) are formed during the spring 

 growth of the x\ lem (Fig. 13-15). Onlv the 

 outermost, or youngest, layer of the xylem 

 continues to carry sap upward through the 

 plant, but the older xylem keeps augmenting 

 the strength of the stem as the load of foliage 



EPIDERMIS 

 ^CORTEX 

 ik— -PHLOEM 



XYLEM 



Fig. 13-14. Diagram showing cross-sectional structure 

 of a 3-year-old pine stem. 



