176 THE LIFE OF THE PLANT 



of pressures in vessels. These valves secure the uniform 

 distribution of water in plants. 



Having seen that water moves through the vessels, 

 and also the reason for this movement, we must try 

 and discover its rate. We proceed as follows : we put 

 the cut end of a branch of the plant to be examined 

 into water which contains a small quantity of some 

 substance the presence of which is easily detected in a 

 plant ; thus, if we cut the stem across at intervals after 

 some time, we can discover the height to which the 

 water had time to rise during the experiment. The 

 very greatest height ever reached in this way is approxi- 

 mately seven feet per hour. 



Evaporation from the leaves, which constantly draws 

 water from the aerial parts of the plant, is therefore the 

 principal reason for the absorption of fresh quantities 

 of water from the root into the stem. But, one must 

 ask, why do we ascribe this function of evaporation to 

 the leaves and not to the stem ? The anatomical 

 structure of the stem gives us the answer to this 

 question. It is only at a very young stage that the 

 stem has a skin or epidermis like that of the leaves ; 

 this very early dies, splits, and falls off, while under- 

 neath it, in the part of the stem called the primary 

 cortex, there is formed a corky tissue. It is called cork 

 because it is strongly developed in a certain species of 

 oak, where it forms the material out of which bottle 

 corks are manufactured. The structure as well as the 

 external appearance of this tissue may vary very much : 

 thus, for instance, in the cork oak it forms a con- 

 tinuous layer several inches thick ; while in the birch- 

 tree it is only a thin and scaly bark. In all cases, 

 however, it has the same general property, namely, 

 that it is impermeable to water, and thereby forms a 

 kind of impervious covering on the stem guarding 

 it from unnecessary or even harmful evaporation. It 

 is a curious fact that this cork tissue spontaneously 



