280 TEXTBOOK OF BOTANY 



continuous from year to year in perennial stems. In time the cells of the 

 pith usually die, and the protoplasm disintegrates. All soluble substances 

 may diffuse out, and onlv the cell walls remain. In stems of some plants 

 (walnut, hackberry, etc.), some of the cell walls also disintegrate or 

 collapse, and the pith becomes chambered. In many species of herbs all 

 the cells of the pith soon disintegrate and the stem becomes hollow. The 

 stems of all these herbs are solid at the tip where the pith is young and 

 intact. 



In perennial plants the pith, or certain cells in it, may live from one to 

 many years: one year or less in the European larch, 10 years in Scotch 

 pine, 17 years in horse chestnut, 27 years in gray birch, and more than a 

 century in a columnar cactus of the American desert. The pith cells in 

 this cactus not only remained alive from 100 to 150 years, but when the 

 pith was wounded some of the cells near the wound divided and fomied 

 new tissues. 



The epidermis. The epidermis of the stem is continuous with that of 

 the leaves and is comparable to it in most respects. The outer wall is 

 usually thick and heavily cutinized. Glaucous stems, like glaucous leaves, 

 are covered with wax. Epidermal hairs are the rule rather than the excep- 

 tion. Stomates are present, though usually not as abundant as in leaves. 

 In many stems they ultimately become ruptured by lenticels that develop 

 from cortical cells beneath them. The first cork cambium that develops 

 in stems occasionally originates in epidermal cells. 



The epidermis of a leaf lives as long as the leaf. This is also true of 

 some stems; but in perennial stems that increase in diameter by cambial 

 growth, the epidermis is soon ruptured, except in those plants where it 

 continues to grow for a few years. 



The cortex. When all types of stems are compared, the cortex is found 

 to be a very diversified region. It consists primarily of parenchyma cells. 

 The outer layer of cells just beneath the epidermis is often referred to as 

 a hijpodermis; the inner layer of cells adjacent to the pericycle is the 

 endodermis. Each of these layers of cells may be parenchyma and differ 

 little if at all from neighboring cells, or they may be easily distinguished 

 by the shape of the cells and thickness of cell walls. The hypodemiis 

 may be composed of ordinary parenchyma cells, sclerenchyma cells, 

 collenchyma cells, or palisade cells similar to those in leaves. 



The first cork cambium in a stem frequently develops from the hypo- 

 dermis. The development of cork from cork cambium is similar, whether 

 it occurs in herbaceous stems, woody stems, roots, or any other plant 



