336 The Living Plant 



plants thrive better in some shade than in full sun; and it is inter- 

 esting to note that man finds it best to temper the light for some 

 of his crops. This is the reason why shading is placed upon green- 

 houses in summer, and why better tobacco is grown under light 

 cotton tents than in full sun, though here the protection given by 

 the tents against hail storms and wind is also important. In 

 Florida, pineapples grow better under a lattice work shade than 

 in full open sun. 



Third, as to the effects of humidity upon growth. A full supply 

 of water in the soil is essential to the process, for this is the source 

 of the water used in swelling the small new cells to their full adult 

 size. But in addition the amount of moisture in the air has an 

 important influence. Most people know that plants grow best 

 on the kind of day we call "muggy," i. e., one in which the air 

 is humid, even to the point of discomfort for us ; and it is a familiar 

 experience that upon such a day the grass of a lawn fairly grows 

 before the eyes. The influence of humidity in promoting growth 

 can also be traced in our graph (figure 123), which shows that in 

 general growth increases with atmospheric humidity. The chief 

 reason for this relation is easily found. Increased himiidity 

 checks transpiration, and therefore leaves in the plant a larger 

 water supply for use in swelling the growing cells. 



Fourth, as to other influences which affect growth. These are 

 few and comparatively unimportant. Electricity, applied ex- 

 perimentally in limited amount, stmiulates growth to a certain 

 extent but in larger amount checks it; but its influence is not 

 wholly separable from that of heat, and the matter is not so very 

 important, since plants are hardly at all exposed to it in Nature. 

 Poisonous substances in soil or atmosphere often stimulate growth 

 a little at first, though ultimately they check it, through the in- 

 jury they do to the living protoplasm. The presence of a little 

 ether in the air seems, however, to promote growth without sub- 

 sequent detriment, though the reason for this effect is not under- 

 stood. The varying pressure of the atmosphere, recorded by the 



