CHAPTER IV. 



PHYSIOLOGY. 



Special observations on the physiology of waterlilies are not numer- 

 ous. That such plants have the usual "senses of direction " geotropism 

 and heliotropism goes without saying. The primary and adventitious 

 roots are all alike highly sensitive to gravitation at first, but the latter 

 soon become more or less passive and run obliquely downward, while the 

 secondary and tertiary roots ramify throughout the soil from its surface 

 to a depth of 30 cm. or more. The peculiar type of root spoken of on a 

 previous page as " contractile " exhibits a striking physiological adapta- 

 tion. Such roots have been found only on young plants of the Lotos and 

 apocarpous groups, excepting A T . flavo-virens, in which adults also have 

 contractile roots. In all of the plants mentioned, however, resting tubers 

 are produced at the close of the season. The object evidently is to draw 

 these tubers well down into the earth for protection against drought and 

 herbivorous animals. For the adult plants, with the one exception named, 

 perish when the flowering season is over, and consequently are not sup- 

 plied with any special contractile organs. N. jiavo-vircnsy^zanzibariensis 

 bears a few contractile roots on the lower part of the tuber of adult plants. 

 This hybrid usually has persistent tubers as in N. flavo-vircns ; its contrac- 

 tile roots, however, show none of the peculiar structure which character- 

 izes those of N. flavo-virens. As a rule it is the lowest (oldest) root from 

 each of the older leaf-bases which is contractile ; in N. lotus one or two 

 upper roots occasionally contract. This lowest root is strongly geotropic, 

 while the others are rather feebly so. It also persists in a living condition 

 after all of its companion roots have completely rotted away. After con- 

 traction it becomes fusiform in shape. The greater density of the cortex 

 in contractile roots indicates that this is the layer where contraction takes 

 place, probably, as Rimbach (1902) has shown for liliaceous plants, by 

 shortening and widening of the cells. Thus we see a strong differentiation, 

 apparently greatest in N. flavo-virens, into nutritive and mechanical roots. 



In stems the great density of tissue in all but the Castalia group, and 

 especially in resting tubers, has already been shown. This is distinctly 

 related to the storage of starch which takes place so largely in such plants. 



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