214 Causes and Course of Organic Evolution 



separate responses are usually compounded in a resultant line 

 of growth, that is nearly or quite the same as for any one indi- 

 vidually. We can prove further, however, that at the very 

 earliest stage of germination from the parent spore and on to 

 maturity a fourth stimulus — the chemotropic one — is in action 

 and is responded to. For, when we attempt to grow such 

 spores on moist bread, potato, or sugar solution, only failure 

 is recorded. The exact kind of food that can chemotactically 

 stimulate the young plant, and cause it to respond in satisfied 

 manner, is absent in the latter substances, but is present in the 

 horse manure. 



But, while such proenvironal action and response usually 

 remain constant for definite plant parts, some structures show 

 an elastic adapta})ility of response that strongly suggests pro- 

 found and exact influencing of the living cell substance by 

 en^dronal stimuli. Thus the case might be cited of the gem- 

 mae of Marchantia^ that are flat bodies developed on a stalk, 

 and that appear almost uniform with each other till they are 

 washed or blo^vii out from their cups. As one side or the other 

 of the flattened body happens to fall next to the soil, so is the 

 future proenvironal response. For it causes the lower side 

 to form absorptive rhizoids and remain pale, the layer above 

 to elongate into interlaced cells, and the superior zone to de- 

 velop tissues in a still more complicated manner, amongst 

 which green or chlorophylloid cells arise. 



Compounding of diverse stimuli, and resulting proenvironal 

 growth, may be said to be the general rule for the organs of 

 vascular plants, though at times also simple stimuli may be 

 effective. Roots generally behave much in the manner already 

 described for the mycelial threads of Pilobolus, and no better 

 examples for experimental study could be selected. By such 

 simple experiments as are ordinarily performed in botanical 

 laboratories, it can be clearly demonstrated that each primary 

 root tip, say of the pea or bean, is simultaneously affected by 

 molic, chemic, lumic, hydric, and mechanic stimuli, as pointed 

 out in the last chapter. Further, when these stimuli are caused 

 to act not in the same direction, as is commonly true for roots 



