178 TEXTBOOK OF BOTANY 



cultivation of soil is that it is thus made more porous and that 

 a supply of air is carried down to a depth at which roots may 

 use it. The draining of swamps and marshes removes the 

 water which fills the crevices of the soil and at the same time 

 draws a supply of air from above to replace the water. In 

 this way, land that could not before be used for growing crops 

 is reclaimed. One often sees dead trees about the edges of a 

 lake which has been raised to an unusual height by a series of 

 very wet seasons or by a dam in the stream below. In such 

 a case, probably several reasons have contributed to the 

 death of the trees ; but one main reason is that their roots have 

 been cut off from an oxygen supply because the soil in which 

 they grow has been flooded with water. The roots died just 

 as we should die if our supply of oxygen were cut off. 



Many plants that live in or partly in water, like the water- 

 lilies, provide for a supply of air to their stems and roots by 

 large passages filled with air that run lengthwise through their 

 stems or leaf -stalks. These air passages also help in making 

 the stems and leaf -stalks lighter, and so in buoying them up 

 in the water. The bald cypresses of the southern United 

 States and Mexico, which live in swamps, have a striking 

 means of obtaining air for their roots. The roots produce 

 pointed, woody projections that grow directly up, sometimes 

 to a height of two or three feet above the surface of the soil. 

 These " cypress knees," which are conspicuous features of 

 southern swamps, help to secure a supply of air for the roots. 

 As a result of the respiration of roots, carbon dioxid is given 

 off to the soil. When carbon dioxid is dissolved in water, it 

 combines with some of the water to form carbonic acid. 

 Thus the soil water about a respiring root contains a weak 

 solution of carbonic acid, and this acid solution seems to help 

 the plant by dissolving some of the soil substances which the 

 plant needs and which would not dissolve in pure water. As 

 we know, it is only dissolved substances that can be absorbed 

 by root hairs. 



