216 Causes and Course of Organic Evolution 



tropic stimulus of the pot-soil caused a bending aside of the 

 apparently firm and thick roots, from the geotropic and the 

 more distant hydrotropic soil stimulus of the ground beyond 

 on which the pot sat, so that these reached and grew into 

 the pot. 



INIany related examples of root stimulation and proenvironal 

 response could be given and later may be quoted in detail in 

 a special volume. All prove that linked-up or resultant pro- 

 environal action is often wholly determined by the relative 

 strength, proximity, or quaUty of stimulus. But some highly 

 suggestive reversals in root response are known and have been 

 investigated, not a few of which the writer has examined prac- 

 tically. Thus the roots of a considerable number of plants, 

 from ferns like Osiminda cinnamomea up through gymnosperms 

 like the swamp cypress (Taxodium distichum) and mono- 

 cotyledons like Phoenix sp. on to various dicotyledons, are 

 capable of gro^^'ing against gravity, unlike other roots on the 

 same individual. These show normal root caps, root hairs, 

 and internal structure as in ordinary roots. In other words 

 their general morphology seems unaltered from the normal. 

 Their energizing relation however is altered, and tliis pro- 

 duces vertically upgrowing roots. Now the change is regarded 

 by most botanists as being due to an important chemotactic 

 response of roots, namely their perception of and response 

 to oxygen supply in the air above, since the amount seems to 

 be insuflScient in or insufficiently absorbed from the surround- 

 ing water or water-logged soil in which these plants grow. 

 So, while some of the roots continue to be stimulated by, and 

 to respond to, gravic, apohelio tropic, and mechanic forms of 

 energy, these aerating ones have so canceled such in favor 

 of the — to them — more important aerotropic or oxytropic 

 growth response. In all such cases, therefore, proenvironal 

 effort and intrinsic linking up of several lines of most satisfied 

 response have resulted in growth of parts of the same organ 

 morj^hologically in two opposite directions. 



A somewhat analogous and yet different case is seen in 

 parasitic roots of the mistletoe. Here the geotropic stimulus 



