334 The Living Plant 



growth. Other things, also, contribute to the result without 

 doubt, such as the commencement of injurious chemical reactions 

 under the higher temperature, and the accumulation of the waste 

 products which are formed faster than they can be removed. 

 But in general the relations existing between temperature and 

 growth are determined by the power of the plant to control the 

 chemical and physical processes concerned. 



Second, as to the effects of light upon growth. At first thought 

 one would suppose that plants must grow best in bright light, 

 since light is essential to the making of their food, which supplies 

 both the material and the energy for their growth; but in truth 

 it is usually more rapid in darkness. This fact is brought out in 

 our graph (figure 123), though here, as is usually the case, the 

 matter is much complicated because the temperature commonly 

 falls so greatly at night as to neutralize any tendency the plant 

 may possess to grow faster at that tune. But when the {empera- 

 ture remains even, as happens at times on warm nights out of 

 doors, and in greenhouses artificially heated, then most plants 

 show a tendency to grow faster in darkness. These are the con- 

 ditions under which the farmer comments on the great growth 

 that his cucumbers, for example, made in the preceding night. 

 Plants make ample food in the day to supply the growth through 

 the night. When, however, plants are kept continually in the 

 darkness for days together, their growth becomes spindling and 

 weak, and their chlorophyll disappears, as our picture will illus- 

 trate (figure 125). The results of such growth are comparable, 

 in general, with the weakening activity of a fever. 



The reasons why plants grow best in the dark are several. A 

 part of this growth consists in that adaptive lengthening (the 

 "drawing" of gardeners) already considered in our third chapter, 

 whereby plants reach up after light. It is well illustrated by the 

 great length of the stems in our picture (figure 125). A part may 

 result from the fact that during the day all other processes are 

 subordinated to photosynthesis, while at night growth has the 



