122 GROWTH 



The same general observations have been recorded for 

 lower plants. Thus Fawcett * found that the radial growth 

 rate of cultures of the fungus Pythiacystes atrophthora remained 

 constant as long as the environmental conditions did not 

 change. With increasing temperature a greater rate of 

 growth is exhibited up to a certain degree; when this is 

 passed, the rate of growth falls off. Thus : 



At 10 C., radial growth rate = 2-5 mm. per 24 hours. 

 "2oC., =6-0 



,, 28 C., ,, ,, = 7'5 ,, ,, ,, ,, 

 33 C., = 2-6 ,, 



Balls f measured the growth in the length of the hyphae 

 of the fungus causing " sore skin " on the cotton plant. The 

 growth rate at different temperatures is what might be ex- 

 pected from van't HofFs law; but at higher temperatures, 

 38 C., there is a decrease in growth followed by complete 

 cessation probably due to the accumulation within the cell 

 of certain products of catabolism ; these deleterious substances 

 presumably are formed at lower temperatures but at a much 

 slower rate; they diffuse out into the surrounding medium, 

 especially at higher temperatures, possibly on account of altera- 

 tions in the permeability of the protoplasm at these higher 

 temperatures.^ In the case of higher plants, the outward 

 diffusion of these harmful bodies, provided they be formed, 

 must be slower on account of the more massive nature of 

 the structures, or they are oxidized within the tissues them- 

 selves. 



As Balls points out, since the conditions under which 

 this decomposition takes place must be fairly uniform in a 

 higher plant, growth optima are shown which are the ex- 

 pressions of the internal struggle between the increasing 

 rapidity of chemical change with the rise in temperature and 

 the inhibitory action of the accumulating products of cata- 

 bolism. 



The investigations of Leitch on the influence of tempera- 

 ture on the rate of growth in the roots of Pisum sativum, 

 show that the relationship can be expressed as a uniform 



* Fawcett : "Johns Hopkins Univ. Circ.," 1917, 193. 



f Balls : " Ann. Bot.," 1908, 22, 557. 



JSee Eckerson: " Bot. Gaz.," 1914, 58, 254. 



Leitch: "Ann. Bot.," 1916, 30, 25. 



