Improvements Made in Plants by Man 431 



which individuals have of adjusting their parts to pecuharities in 

 the distribution of hght, moisture, minerals or other essential 

 conditions of their immediate surroundings, — a power which we 

 have studied with some care in the chapter on Irritability. An 

 example thereof is the greater lengthening of stems (which are 

 "drawn," in the language of gardeners) when exposed to insuffi- 

 cient light, and of which the very long shoots formed by potatoes in 

 cellars are an extreme example. Upon these stems the deficiency 

 of light acts as a stimulus, making them lengthen out as if in the 

 effort to carry their leaves into full brightness ; and man can make 

 plants long-stemmed in this way if he wants to. But when all of 

 the differences due to mechanical causes or to irritability are 

 eliminated, as can largely be done by careful experiment, there 

 remains always a great residue of differences for which there is no 

 conceivable origin except that they are innate or inborn in the 

 plants themselves. Thus, when all external conditions of water- 

 supply, light, &c., are carefully made the same for all the plants 

 under our experiment above described, the stems of the plants 

 nevertheless differ in length, some being shorter and some longer. 

 And man can also obtain long-stemmed plants in this way if he 

 wants to. It is thus plain that the differences between individual 

 plants of the same species arise in at least three different ways, 

 which we may summarize in an order the reverse of that of their 

 treatment above, by saying, — that in some of their features plants 

 are born different, others of their differences are achieved, while 

 some of their differences are thrust upon them. 



Of the three kinds of differences, the inborn variations are the 

 only ones important in the improvement of plants, as in natural 

 evolution, and for the same reason, that they are the only kind 

 which can be transmitted to descendants. Although man is able 

 by regulation of the water or light supply to make individual 

 plants short-stemmed or long-stenmied, he cannot by this means 

 make a short-stemmed or long-stemmed race which will reproduce 

 itself generation after generation. The only known way in which 



