24 



THE FACTORS 



[Part I 



The directions in which the modifying influence of water has chiefly 

 acted are as follows : enlargement of the external surface of the shoots by 

 the lengthening or subdivision of their members, suppression or diminution 

 of the root-system along with, it may be, its transformation into fixing 

 organs, weak development of the cuticle, absence or reduction in number of 

 the stomata, displacement of the vascular bundles to form a central strand, 

 peripheral disposition of the chlorenchyma. In still waters there is 



besides, as a rule, a diminution 

 in the mechanical elements and 

 an increase in the air-containing 

 intercellular spaces. These mo- 

 difications are not so noticeable 

 in actively moving water, where 

 also the relatively large ex- 

 tension of surface is less pro- 

 nounced. The utility of these 

 modifications is obvious. It is 

 only doubtful whether they are 

 to be attributed to natural selec- 

 tion or to the direct influence 

 of water. The first hint in 

 explanation of some of them 

 comes from the Cardamine ac- 

 cidentally growing in water, in 

 regard to which there can be 

 no question of natural selection. 

 Probably both groups of influ- 

 ences, the direct and the indirect, 

 have acted simultaneously. 



The modifications which ex- 

 istence in water induced in 

 plants that were originally ter- 

 restrial are only in part attri- 

 butable to the direct action of 

 water. For the rest, it is a 

 question of other factors of 

 plant-life, which are themselves 

 modified by water. Some characteristics of aquatic plants are to be 

 attributed to the weakening of the light in water, and they accordingly 

 reappear in terrestrial plants growing in deep shade ; in illustration of this 

 may be cited the peripheral arrangement of the chlorenchyma and 

 possibly the great lengthening of parts in deep water. The considerable 

 extension of the plant-surface, and the abundance of air-containing canals 



Fig. 30. Ranunculus fluitans. 1. Aquatic form. 

 2. Terrestrial form. Two-thirds natural size. 



