CHANGES IN THE BATE OF GROWTH. 389 



question is not yet positively answered by any experiments. It 

 has been shown that some plants grow, for a time at least, more 

 rapidly when they are subjected to a slight increase of pressure of 

 the atmosphere by which they are surrounded ; but there are also 

 a few cases which indicate that some other plants may grow more 

 rapidly under a diminished pressure. 



The " resting" state of some plants cannot be shortened by 

 an}' increase in the amount of oxygen furnished ; it is only after 

 the normal time of rest has ended that any growth begins. 

 When periods of rest cannot be disturbed by any ordinary change 

 in the surroundings, they ma}* be held to be conservative, since 

 they are generally correlated with the climatic conditions of 

 peril from cold or from dryness, under which these plants 

 naturally live. 1 



1019. Periodical changes in the rate of growth. Even under 

 external conditions which are as nearly constant as possible 

 growth is not quite uniform in its rate. Thus, an extending 

 internode grows in length at first slowly, then with gradually 

 accelerating rapidity until a maximum of growth is reached, 

 from which point the rate declines until with maturity of the part 

 growth ceases. The line of growth, when given graphically, is 

 a curve known as the great curve of growth ; and the period of 

 rise and decline is the grand period, to distinguish this from the 

 minor periods of accelerated growth, which appear on the curve 

 as small fluctuations. 



1020. Properties of new cells and tissues. Newly formed cells 

 are generally characterized by the possession of a certain amount 

 of turgidity ; the young cell- wall exerting more or less resistance 

 to the expansive contents within. The contents are therefore 

 compressed to some degree by the confining wall ; the action 

 and reaction varying, of course, with changes in the surroundings. 

 If a part of its water be withdrawn from the cell, the com- 

 pression is materially lessened ; while, on the other hand, an 

 increase in the amount of water must augment it. 



1021. These features have been recently re-examined by De 

 Vries, who has suggested a quantitative method for determining 

 the amount of turgidity at an}* given time. The method, when 

 reduced to its simplest terms, consists in the use of solutions of 



1 For a very curious account of experiments upon the influence of electricity 

 upon growth, the student should see Grandeau : De 1'influence de 1'electricite 

 atniospherique sur la nutrition des vegetaux, Annales de Chimie et de Phy- 

 sique, ser. 5, tome xvi., 1879, p. 145. 



