CH. IV, 7] GROWTH OF PLANTS 157 



collectively a continuous record. The grand period is 

 most familiarly manifest in the opening out of the spring 

 vegetation, in which, after a slow expansion of the buds, the 

 actual opening takes place very rapidly, and full size is soon 

 reached. In general the spring vegetation opens out on the 

 crests of the grand periods of the parts concerned. 



As to the minor fluctuations of the growth records, they 

 are found by experiment to have precisely the meaning 

 which one naturally ascribes to them, viz. they are con- 

 nected with the weather. If careful comparison be made 

 between rate of growth and the contemporaneous mete- 

 orological conditions, the following general results become 

 evident. 



1. Higher temperature promotes growth, and lower checks 

 it. This fact is of course sufficiently familiar, for everybody 

 knows that plants grow faster in warm weather and slower 

 when cool. The reason thereof is this, growth involves 

 a number of physical and chemical processes, all of which 

 are directly promoted by heat. However, so far as plants 

 are concerned, there are limits to its favorable action, be- 

 cause above ordinary temperatures heat begins to act in- 

 juriously upon the protoplasmic constituents, especially 

 the susceptible proteins, which easily coagulate. Each 

 plant has a minimum temperature below which it does not 

 grow at all, an optimum temperature at which it grows fast- 

 est, and a maximum temperature beyond which it ceases to 

 grow. The conventional constants for these cardinal points, 

 for our common plants collectively, are 5 - 30 - 40 C. 

 The matter is illustrated very graphically when plants are 

 grown in a differential thermostat (Fig. 110), in which they 

 can be made to plot their own curve, so to speak, of their 

 growth in relation to temperature. By use of this instru- 

 ment the three points may be determined for any given 

 plant, and thus it is also shown that in general the points 

 range higher in tropical plants, lower in the arctic kinds, and 

 intermediate in those of temperate regions. Since the 



