THE OUTER TISSUES OF PLANTS. 255 



branches. But 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 rhizome 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 completely changed to fit 

 under-ground actions, that they will not resume aerial func- 

 tions; and under-ground organs so completely changed to 

 fit aerial actions, that they will not resume under-ground 

 functions. 



That the physiological differentiation between 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 



* A critical comment made on this sentence runs as follows : — " The 

 aerial roots of most epiphytic orchids contain chlorophyll in their cortex 

 throughout their length, but the cortex being covered by a l velamen ' of air- 

 containing cells which break up and reflect incident light, the green colour 

 is not visible through this opaque coat. When moistened the cells of the 

 velamen take up water and the green colour immediately shows through. 

 Such roots do not however possess stomata. The roots of certain species of 

 Angrozcum, however, contain the whole of the assimilating tissue of the 

 Dlant." 



