CAUSES OF VARIATION 



Microscopical examination showed that in the dry-air specimens 

 " the palisade cells were well developed and there was a special 

 consolidation of fibrous tissues." l The same author continues : 



Again, the common water reed Phragmites communis, when growing in 

 the unirri gated areas of the Nile valley, forms a stunted growth with very 

 short and sharp-pointed leaves. Close to the Nile, however, ... it grows 

 nine or ten feet high, with long leaves almost exactly like the plants in 

 English rivers. 



All observations go to show that the number of vessels in the 

 fibre-vascular system is greater in the aerial than in the aquatic 

 forms of the same species, 2 and the evidence in general seems 

 conclusive that the notorious abundance of spines in tropical 

 vegetation is due primarily to a dry atmosphere, assisted to 

 some extent, no doubt, by the retarding effect of intense light 

 upon growth. 



A significant fact in this connection, possibly attributable to 

 the scarcity of water, possibly to the lack of heat, is the well-known 

 phenomenon that plant lice, producing females only during the 

 summer, begin with approaching autumn to produce males, and 

 that under the perpetual heat of the greenhouse the insects ob- 

 serve summer habits indefinitely tmless tJie plants on which they 

 are feeding are allowed to become dry. 



As is well known, seeds may be kept for long periods if 

 thoroughly dried. In this case the vital activities are reduced to 

 a minimum, but probably not entirely suspended, because seeds 

 will not last indefinitely. Certain lower forms of plant and ani- 

 mal life have a marked power of apparently suspending life 

 through desiccation and resuming its activities again with suffi- 

 cient moisture. 3 



The actual influence of water upon development is not yet 

 well understood, except that it is one of the absolute conditions 

 of life, and, being a fluctuating element, often limits development. 



The student should fully appreciate the bearing of all this upon 

 the matter in hand : The degree of development of an individual 

 at maturity is not a complete index to his inherited characters. 



1 Vernon, Variation in Animals and Plants, p. 264 ; see also Henslow, Origin 

 of Plant Structures, p. 40. 2 Vernon, Variation in Animals and Plants, p. 265. 



3 C. B. Davenport, Experimental Morphology, Part I, pp. 59-65. 



