270 PLANT-LIFE 



This purely physical process is readily illustrated by a 

 simple experiment. A glass tube is closed at one end 

 by a piece of bladder, and partially filled with a solution 

 of common salt; the closed end of the tube is now 

 inserted in distilled water, and in due course the solution 

 in the tube will be observed to rise. The salt in the 

 tube has attracted the water, which has made its way 

 into the tube through the permeable bladder. Absorp- 

 tion by root-hairs takes place on exactly the same 

 principle. The cell-sap contains salts in solution and 

 organic acids ; it is denser than the solution contained in 

 the soil, and attracts it even as the salt solution attracts 

 the distilled water. The cell-wall is permeable, so the 

 external weak solution is drawn through it to the inside 

 of the cell, and it is passed on from cell to cell throughout 

 the plant by repeated osmosis. 



For some account of the great importance of carbon 

 as a plant food, and the manner in which it is ab- 

 sorbed in the form of carbon dioxide by green plants, 

 and assimilated with the assistance of chlorophyll, 

 the reader is referred to p. 19. To recapitulate the 

 story here would be a work of supererogation, but it 

 must be borne fully in mind, otherwise the student 

 will have a totally inadequate conception of plant 

 nutrition. 



In considering the physiology of plants it is further 

 necessary to realize that, like animals, they breathe, 

 the breathing process being termed " respiration." The 

 process is certainly not so obvious in plants as it is in 

 the higher animals, yet it most assuredly takes place. 

 In breathing an animal takes in oxygen and exhales 

 carbon dioxide as a product of internal combustion in 



