78 THE STORY OF THE PLANTS. 



needful solvent, but also other materials urgently 

 required for their growth and development. The 

 most important of these materials is certainly 

 nitrogen, which forms an indispensable com- 

 ponent of protoplasm and chlorophyll. Where, 

 however, the roots do not supply nitrogenous 

 matter in sufficient quantities, plants procure it 

 for themselves by means of their leaves or stems, 

 and therefore become insect-eating or flesh- 

 eating. Soils get exhausted at times of nitrates, 

 phosphates, and other necessary materials of 

 plant-life. The farmer meets this difficulty by 

 manuHng, and by rotation of crops. Nature 

 meets it by dispersion of seeds. Koots, however, 

 have other functions besides drinking water and 

 sucking up with it certain dissolved materials^ 

 the chief of these other functions are fixing the 

 plant securely in the ground, and affording a 

 safe place of winter storage for starches and 

 other surplus food-stuffs. Many plants die down 

 almost entirely, above ground, in winter, 

 and keep their raw material in underground 

 reservoirs, most of which are stem-like rather 

 than root-like. Animals, however, find out these 

 subterranean reserves, and prey upon them ; 

 hence the plants often secure their hoard by 

 nauseous tastes or other protective devices. 



CHAPTEK VI. 



HOW PLANTS MARRY. 



We next come to what is perhaps the most 

 fascinating chapter of all in the life-history of 



