238 PHYSIOLOGICAL DEVELOPMENT. 



branches. Bat the most conclusive evidence is 



furnished by the actual substitutions of surface- structures 

 and functions, that occur in aerial organs which have taken 

 to growing permanently under ground, and in under- ground 

 organs which have taken to growing permanently in the 

 air. On the one hand, there is the rhizoma, exemplified by 

 Ginger — a stem which, instead of shooting up vertically, 

 runs horizontally below the surface of the soil, and assumes 

 the character of a root, alike in colour, texture, and 

 production of rootlets ; and there is that kind of swollen 

 under- ground axis, bearing axillary buds, which the Potato 

 exemplifies — a structure which, though homologically an 

 axis, simulates a tuberous root in surface-character, and 

 when exposed to the air, manifests no greater readiness 

 to develop chlorophyll than a tuberous root does. On the 

 other hand, there are the aerial roots of certain Orchids, 

 which, habitually green at their tips, continue green 

 throughout their whole lengths when kept moist ; which 

 have become leaf-like not only by this development of 

 chlorophyll, but also by the acquirement of stomata; and 

 which do not bury themselves in the soil when they have 

 the opportunity. Thus we have aerial organs so com- 

 pletely changed to fit under- ground actions, that they will 

 not resume aerial functions ; and under-ground organs so 

 completely changed to fit aerial actions, that they will not 

 resume under- ground functions. 



That the physiological differentiation betweon the part of 

 a plant's surface which is exposed to light and air and the 

 part which is exposed to darkness and moisture and solid 

 matter, is primarily due to the unlike actions of these 

 unlike parts of the environment, is, then, clearly implied by 

 observed facts — more clearly, indeed, than was to be expected. 

 Considering how strong must be the inherited tendency 

 of a plant to assume those special characters, physio- 

 logical as well as morphological, which have resulted from 

 an enormous accumulation of antecedent actions, it may 



