158 



THE INDIVIDUAL ORGANISM 



the first. Each successively deeper layer causes the death of all cells 

 outside it by cutting off their supply of water and food. This process 

 converts more and more of the original cortex into dead outer bark, and 

 results in the formation of a thick protective covering, the surface of 

 which scales off or is fissured into ridges or plates as the trunk increases 

 in diameter from within. When the zones of cork formation finally invade 

 the outer and older layers of the phloem, the trunk comes to consist 

 solely of the stele. New layers of phloem, continually formed by the 



cambium, replace those lost to the 



outer bark. 



Other types of stem structure. The 



kind of stem just described is that 

 characteristic of the woody dicotyle- 

 dons, typified by the hardwood trees. 

 The pines and their allies (gymno- 

 sperms) have a very similar type of 

 stem. It differs from the hardwood 

 type chiefly in that the conducting 

 tissue of the xylem is composed almost 

 entirely of tracheids, without vessels 

 but generally with resin ducts, and that 

 the sieve tubes of the phloem have no 

 companion cells. (Tracheids, vessels, 

 sieve tubes, and companion cells are 

 described on pages 160-161.) 



Herbaceous dicotyledons, such as bean 

 and thistle, have stems built on the same 

 general plan as those of the woody dicot- 

 yledons. The amount of vascular tissue 

 is relatively less and the pith and cortex are relatively greater in amount, and the 

 vascular ring is generally broken by pith rays into a number of separate vascular 

 bundles arranged in a ring around the pith. Each of the vascular bundles is like a 

 segment of the vascular ring of the typical woody dicot stem; it contains a cam- 

 bium layer, separating an outer group of phloem cells from an inner group of 

 xylem cells. Because of the presence of the cambium these bundles are capable 

 of growth and are hence called open bundles, in contrast to the closed bundles of 

 the monocotyledons. 



Monocotyledons (for example the grasses, maize or corn, lilies, and palms) 

 have a type of stem (Fig. 11.6) very different from the other two sorts. The 

 meristematic zone in the growing point is almost as great in diameter as the 

 mature stem. It gives rise to an outer epidermal layer, which encloses a narrow 

 zone that corresponds to the cortex and pericycle of the dicot stem; this zone is 

 often strengthened with mechanical tissue to form a supporting cylinder, the 

 position of which gives maximum strength for amount of structural material used. 

 The rest of the interior of the stem tip is made up chiefly of parenchyma cells, 



Fig. 11.5. Cross section of the outer part of 

 a young stem of basswood (Tilia), showing 

 annual layers of xylem and the broken 

 phloem segments outside the cambium 

 layer. The spaces between the phloem 

 columns are occupied by parenchyma. 

 (Photo courtesy of General Biological Supply 

 House, Inc.) 



