66 The Living Plant 



but quite as thick as a house in the larger species of Redwood; 

 branched to a spray hi a Mango Tree, but an unbranched shaft 

 in the Royal Palm. Thus it is evident that leaves and stems ex- 

 hibit well-nigh as remarkable a diversity in size as in shape, and 

 we must conceive of our generalized or composite leaf and stem 

 as well-nigh indefinitely modifiable, possessing, as it were, a 

 kind of a super-elasticity in both of these features. As to the 

 causes determining size in these parts, that is reserved for dis- 

 cussion in the chapter on Protection, where it will be shown that 

 the size actually displayed by any leaf or stem represents in the 

 main a compromise or truce between the conflicting tendencies 

 of the plant to make its leaves larger for photosynthetic advantage 

 on the one hand, and smaller for better resistance to hostile ex- 

 ternal conditions on the other. 



In this chapter thus far but little has been said concerning the 

 root. This is because the consideration of that organ is more 

 convenient and natural in the chapter that deals with its function 

 of Absorption; and there its description will be found in detail. 

 It is enough for our immediate purpose to say that roots, the 

 principal organs for the absorption of water and minerals, and 

 the third of the primary plant parts, grow out from stems, which 

 they closely resemble in structure, having much the same internal 

 cellular construction as well as the same long-tapering, freely- 

 branching forms. Though not without diversity in form, size, 

 and structure, they are yet far less varied in these respects than 

 are leaves and stems, and for a sufficient and obvious reason, 

 namely, they grow under far more uniform conditions; for life 

 in the soil is much the same thing all the world over, however 

 varied it may be upon the surface. 



Thus far we have considered only those diversities which leaves 

 and stems exhibit while still retaining their typical function of 

 photosynthesis. But their remarkable plasticity does not exhaust 

 itself here, for these parts can even perform entirely different 

 functions, becoming adaptively modified therefor to such a de- 



