158 A TEXTBOOK OF BOTANY [On. IV, 7 



cardinal points are ingrained in the protoplasm of particular 

 species, it is clear why tropical plants do not thrive when 

 taken abruptly to the arctics, and vice versa. The points, 

 however, are alterable somewhat with time, as manifest in 

 acclimatization. 



2. Humidity in the atmosphere (distinct altogether from 

 the water supply at the roots) promotes growth, while 

 dryness checks it. Everybody knows that plants grow best, 

 on the whole, during close, " muggy" days, the kind most 

 uncomfortable to us; and such days are called "growing 

 days" by some farmers. The reason thereof is chiefly this, 



FIG. 110. Effect of different temperatures upon Oats grown in a dif- 

 ferential thermostat, an instrument evenly heated from the right side and 

 cooled from the left ; X t^- (Drawn from a photograph.) 



that the enlargement of young parts is forced by the in- 

 ternal pressure of water (the osmotic pressure explained in 

 the next chapter) ; this internal pressure is lessened by re- 

 moval of much water through high transpiration caused by 

 dry air, but is maintained with low transpiration accompany- 

 ing humidity of the atmosphere. Other things being equal, 

 the growth of vegetation is more rapid in regions of moist 

 climate, and slower in dry ones, as forests and deserts respec- 

 tively illustrate. The matter, however, is complicated by 

 the matter of water supply to the roots, and other consid- 

 erations. 



3. Light affects plants complexly. Indirectly, of course, 

 it is essential to green plants through its relation to food sup- 

 ply. Directly, however, it has no great effect unless very 

 strong, when it tends to check the growth of most plants, 

 which can thrive best under some shade, as we have earlier 

 noted (page 95). The records of auxographs show very 



