300 PLANT STRUCTURES 



In such plants the carbon dioxide is absorbed directly 

 from the air by the foliage leaves, whose expanse of surface 

 is as important for this purpose as for exposing chlorophyll 

 to light. When the work of foliage leaves is mentioned it 

 must always be understood that it applies as well to any 

 green tissue displayed by the plant. 



The water, with its dissolved salts, is absorbed from the 

 soil by the roots. Only the youngest parts of the root- 

 system can absorb, and the absorbing capacity of these 

 parts is usually vastly increased by the development of 

 numerous root hairs, just behind the growing tip (Fig. 194). 

 These root hairs are ephemeral, new ones being continu- 

 ally put out as the tip advances, and the older ones disap- 

 pearing. They come in very close contact with the soil 

 particles, and "suck in" the water which invests each 

 particle as a film. 



162. Transfer of water. The water and its dissolved salts 

 absorbed by the root-system must be transferred to the foli- 

 age leaves, where they are to be used, along with the carbon 

 dioxide, in the manufacture of food. 



Having entered the epidermis of the absorbing rootlets 

 the water passes on to the cortex, and traversing it enters 

 the xylem system of the central axis. In some way this 

 transfer is accompanied by pressure, known as root pres- 

 sure, which becomes very evident when an active stem is 

 cut off near the ground. The stump is said to "bleed," 

 and sends out water ("sap") as if there were a force 

 pump in the root-system. This root pressure doubtless 

 helps to lift the water through the xylem of the root into 

 the stem, and in low plants may possibly be able to send it 

 to the leaves, but for most plants this is not possible. 



When the water enters the xylem of the root it is in a 

 continuous system of vessels which extends through the 

 stem and out into the leaves. The movement of the ab- 

 sorbed water through the xylem is called the transpiration 

 current, or very commonly the "ascent of sap." An ex- 



