PLANT PHYSIOLOGY 299 



159. Stereome. When growth is completed, cell walls 

 lose their elasticity, turgidity becomes less, and therefore 

 tensions diminish, ard rigidity is supplied by special ster- 

 eome tissues, chief among which is sclerenchyma. An- 

 other stereome tissue is collenchyma, which on account of 

 its elastic walls can be used to supplement turgidity and 

 tension where elongation is still going on. For a fuller 

 account of stereome tissues see 8 150. 



160. Food. Plant food must contain carbon (C), hydro- 

 gen (H), oxygen (0), and nitrogen (N), and also more 

 or less of other elements, notably sulphur, phosphorus, 

 potassium, calcium, magnesium, and iron. In the case 

 of green plants these elements are obtained from inor- 

 ganic compounds and food is manufactured ; while plants 

 without chlorophyll obtain their food already organized. 

 The sources of these elements for green plants are as 

 follows: Carbon from carbon dioxide (C0 2 ) of the air; 

 hydrogen and oxygen from water (H 2 0) ; and nitrogen 

 and the other elements from their various salts which 

 occur in the soil and are dissolved in the water which 

 enters the plant. 



All of these substances must present themselves to 

 plants in the form of a gas or a liquid, as they must pass 

 through cell walls ; and the processes of absorption have 

 to do with the taking in of the gas carbon dioxide and of 

 water in which the necessary salts are dissolved. 



161. Absorption. Green plants alone will be considered, 

 as the unusual methods of securing food have been men- 

 tioned in Chapter VII. For convenience also, only terres- 

 trial green plants will be referred to, as it is simple to 

 modify the processes to the aquatic habit, where the sur- 

 rounding water supplies what is obtained by land plants 

 from both air and soil. 



