itOOTS -J.'.] 



30. Relation of earth roots to air and water. The soil at 

 moderate depths contains much air in its pores, the amount 

 being largest in light loams and sand, and least in stiff clays. 

 This air is essential to the health and growth of ordinarj- 

 roots. Many kinds of plants growing in earth are quickly 

 killed when transferred to a glass battery jar with a lead cover 

 sealed on, if water enough is poured in through a thistle tube 



Fii;. 19. Cypress trees (T<i:codlum) growing in a swamp 



The conical " knees " growing from the roots and nearly always ahove water are 

 thought to serve as channels to supply air to the roots 



to fill the jar almost to the exclusion of air. In the same way. 

 if water is backed up a stream when a dam is built across it, 

 most of the trees that are surrounded by the pond formed by 

 the retained water are killed. They have been drowned, and 

 die for lack of au-. Even the lower forms of green plants 

 (Figs. 156-168) will soon die for lack of it, if kept in a 

 tightly stoppered jar or bottle full of water. Most aquatic 

 plants which have leaves or green stems exposed to the air — 

 like pond lilies, some rushes, cat-tails, and so on — convey air 

 down into the submerged parts by means of numerous 



