EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



231 



Water, or the lack of it, is therefore in many countries and in 

 many seasons the limiting element ; that is to say, the yield is 

 limited not by the available fertility or the ability of the crop but 

 by the moisture present. 



In excessively wet seasons crops are notoriously " soft," that is, 

 lacking in substance. Just what the difference is has not been 

 well established, but size has been attained at the expense of 

 quality. There is good reason to assume that it is the result of 

 abundance of water, leading to full cellular development, but 

 deficiency of evaporation and transfusion of food due to cloudy 

 skies, resulting in a lack of actual dry matter. 



Effect of moisture in the atmosphere. It is said that moist 

 atmospheres produce fineness of hair or fur in animals and deli- 

 cate foliage in plants, and that a dry atmosphere inclines to a 

 harsh, dry coat and to spiny growth in plants. Under natural 

 conditions, however, moisture is often associated with coolness and 

 shade, and dryness with great heat and intense light. Certain it 

 is that fur-bearing animals are found in cool climates and that 

 vegetation is delicate in the temperate region but harsh, dry, 

 and spiny in the arid sections. These facts are well known and 

 universally recognized, but how much is due to moisture alone 

 cannot well be determined in nature. 



Resorting to direct experiment, however, we find that the same 

 plant may be grown with or without spines according to the 

 degree of moisture in the surrounding atmosphere. Spines are 

 undeveloped leaves, as thorns are abortive stems, and anything 

 that checks growth tends to their production. That this is mainly 

 the result of a dry atmosphere, however, is easily shown in the 

 laboratory. 



" Lothelier has made numerous observations in which individ- 

 uals of the same species were placed side by side, some exposed 

 freely to the air and others kept moist under a glass shade." 

 Under conditions such as this " Berberis vulgaris bore non- 

 spinescent leaves in a moist atmosphere, but spines alone in a 

 perfectly dry one. Again, the shoots which in Lycium barbarum, 

 Ulex Europcens, etc., would normally have formed thorns, by 

 arrested development and sclerosis (hardening), in a very damp 

 atmosphere continued to grow, and elongated into leafy branches." 



