128 A TEXTBOOK OF BOTANY [Ch. IV, 3 



(Fig. 72), and partly on the perfection to which the hollow- 

 column principle is carried in their construction, as witness 

 the Bamboo. Upon the latter feature they depend far more 

 than do exogenous plants, which find ample support in their 

 massive solid trunks. Some Monocotyledons, however, do 

 exhibit increase in diameter, for the outer layers of their 

 stems develop a cambium-like tissue which continues to 

 form new scattered bundles as long as the plant lives. It is 

 thus that the great Dragon Tree, though endogenous, can 

 attain to so great a diameter and age (Fig. 84). In all endog- 

 enous plants, the seeming bark is nothing other than the 

 compact outer tissues, darkened more or less by action of 

 the weather, of which the effects penetrate to some depth. 



Striking though the difference appears between the 

 exogenous and endogenous types of stems, they perform the 

 same functions with apparently equal efficiency. The 

 differences between them are therefore not functional, but 

 depend rather upon their relationships within two dif- 

 ferent and ancient lines of evolutionary descent. Did we 

 not know this fact, we might seek long for a functional ex- 

 planation of differences the significance of which lies only in 

 heredity. 



3. The Cellular Anatomy of Stems 



From the tissues of stems, which can readily be recognized 

 by aid of a hand lens, we turn naturally to consider the 

 constituent cells, making use of the microscope. 



One of the best stems for such study, because of its ex- 

 ceptionally clear definition of the parts, is that of the Dutch- 

 man's Pipe (Aristolochia Sipho), a common vine. Sections 

 through the terminal bud, or very close thereto, show only 

 the closely packed, squarish, protoplasm-filled cells which 

 one soon learns to associate with the embryonic stage of 

 growth (compare Figs. 92, 162). Such embryonic tissue is 

 always called meristem, whether in buds, growing tips of 

 roots, cambium, or elsewhere. A little behind the bud 



