32 THE STORY OF PLANT LIFE 



of land-plants, the air being diffused in water and 

 the nutrient salts in the water. 



Oxygen in either case, necessary for respiration, is 

 derived from air by land-plants, from water alone by 

 water-plants — that is, entirely submerged aquatics in 

 all the cases mentioned above. Aquatic plants that 

 are partly submerged, partly aerial, are intermediate 

 in these respects. 



Nutrition depends on the formation of organic 

 food or carbohydrates in the plant, by the process of 

 assimilation, and of proteids, etc., derived by absorp- 

 tion of mineral salts in solution, and transformed into 

 organic substances in the plant. 



The water absorbed is necessary to the plant, and 

 forms a large part of its substance as cell-sap, but 

 cannot be considered as food, though necessary for 

 the proper carrying out of the process of transpira- 

 tion, for carbon assimilation, etc. 



Very little free nitrogen is obtained from the air by 

 plants, but it is secured indirectly by leguminous 

 plants from the bacteroids in their root-nodules. 



A young seedling is already supplied with reserve 

 material in the endosperm with which the embryo is 

 frequently provided. When the radicle and hypo- 

 cotyl have elongated, and the seed leaves are exposed 

 to the light and air, the various processes already 

 mentioned, absorption, transpiration, respiration and 

 assimilation commence, and the young plant is 

 enabled to carry on the functions of nutrition. 



As soon as certain conditions are realised, a suitable 



