LEAVES IN SEASONS 157 



through the membranous walls into the air- 

 spaces, and passes outward through the 

 breathing pores (stomata) into the open air 

 in the form of vapor. The quantity of water 

 poured into a thirsty sky in the heat of a mid- 

 summer day is by no means inconsiderable, 

 even in smaller plants, and in a full-grown 

 poplar tree may amount to a barrel. As the 

 water enters the roots it contains from one- 

 ten-thousandth to a thousandth part of its 

 weight of potassium, calcium, and magnesium 

 salts in solution, and it evaporates into the 

 air leaving the mineral compounds in the 

 plant. The minerals serve important uses in 

 building up protoplasm, and facilitating the 

 diffusion of food substances from one part of 

 the plant to another. Eventually a large 

 proportion of these substances accumulate in 

 layers on or in the cell wall, or as crystals in 

 the cell cavity, particularl3^ in the leaf, in such 

 condition as to be of but little use to the or- 

 ganism, and it would be benefited by being 

 freed from this superfluous matter. Besides 

 the inward condition of the leaf, changes in 

 the environmental conditions make it highly 

 important that the plant should dispense 

 with its leafy extensions. 



With the approach of the close of the grow- 

 ing season, the outward conditions have un- 

 dergone a gradual and thorough change and 



