132 Outline of Genetics 



would wash the pollen grains off the stigmas, and thus result in a 

 certain amount of sterility.) 



2. Conditions too dry. 



Unusual drought at the period that the stigmas are receptive 

 may dry the stigmatic surfaces to such a degree that pollen grains 

 will not adhere and germinate. Conditions of this sort at times 

 limit the setting of seed in such plants as corn. 



I and 2 provide an illustration of a principle which is frequently 

 encountered in biology^ — -opposite extremes of conditions bringing 

 a similar end result (although the intervening sequences of causes 

 and effects will of course differ). Other illustrations of this same 

 thing appear later in this classification. 



3. Poor ''nutrition." 



This may so limit the development of plants that they fail to 

 mature up to the point of effecting a sex act. 



4. Good ''nutrition." 



A principle familiar to botanists is that the optimum growth 

 conditions frequently maintain the plant in the purely vegetative 

 phase, so that reproductive parts are not developed. (The plant 

 physiologist describes this in terms of a carbohydrate : nitrogen 

 ratio.) 



5. Season too short. 



Plants adapted to a long growing season are unable to com- 

 plete their normal life-cycle up to the point of successful reproduc- 

 tion when grown in regions which have a short season. 



6. Unusual light conditions. 



Flowering and fruiting of many kinds of plants is induced by 

 exposure to specifically favorable length of day which varies widely 

 with the species (see Allard and Garner i). Radical departures 

 from the customary seasonal program in this matter may serve 

 to inhibit flowering and fruiting. (It may be that the underlying 

 causes involved here are similar to those of 3 and 4.) 



Other examples of environmental causes for sterility could 

 doubtless be provided by the plant ecologist. 



II. Large evolutionary tendencies. 



A. Sterility. 



Within certain groups of plants, what is apparently the 

 natural phylogenetic sequence of genera and species indicates 



