120 THE PRINCIPLES OF AGRICULTURE 



temperatures. Such plants as cotton and the 

 melon are killed by a temperature several degrees 

 above freezing. The living protoplasm is stimu- 

 lated to give up its water, the roots are chilled 

 and cannot supply to the leaves that water nec- 

 essary to offset transpiration, and, as a result, 

 the leaves soon wilt and blacken. On the other 

 hand, even the green parts of some plants will 

 withstand freezing temperatures. The ability to 

 resist cold depends primarily on the response 

 of the protoplasm, its capacity to give up water in 

 freezing without injury, together with the power 

 of reabsorption on thawing. 



3. The Processes of Growth 



203. The starch that may result from photo- 

 synthesis or the use of carbon dioxid is stored in 

 the leaves during the day, and at night it may be 

 entirely removed and used after being converted 

 into a soluble substance, sugar. Some of this 

 sugar is directly used in building up more complex 

 compounds used' in growth, and some of it is again 

 converted into starch and stored in tubers, stems, 

 or thickened leaves, for future growth purposes. 



204. The external evidences of growth are 

 changes in form and size of the different parts. 

 The internal evidences of growth are to be seen in 

 the differentiation of the individual cells of which 



