88 Elementary Botany 



stomata. The amount which is evaporated depends upon 

 certain conditions, viz. : — 



a. The state of the atmosphere. All other things being 

 equal, more moisture will be given off in dry than in moist 

 atmosphere. 



b. The amount of light. The greater the light, the greater 

 the transpiration. 



c. The structure of the epidermis. In many succulent 

 plants the epidermis is very thick, thus preventing an excess of 

 evaporation. 



d. There is a greater evaporation from the lower than the 

 upper sides of the leaves. 



The amount of water transpired is often very great, as is 

 shown by experiments which were first performed by Hales in 

 1724. A Sunflower plant, 35 feet high, weighing 3 lbs., and 

 with a surface of 5,616 square inches, exhaled a pint of water 

 a day ; a Cabbage plant with 2,736 square inches, 19 fluid 

 ounces ; a Lemon tree with 2,557 square inches, 6 fluid 

 ounces. 



The action of Wardian cases depends upon this continual 

 evaporation. By this means the water which evaporates col- 

 lects on the glass, and running down once more, waters the 

 plants. 



The transpiration of plants also plays an important part in 

 determining the humidity of the atmosphere where they grow. 

 We always find that if there are forests of trees the atmosphere 

 in the neighbourhood is moist ; whilst, on the other hand, 

 there have been numerous cases where the clearing of a tract 

 of country has materially interfered with the rainfall, producing 

 droughts. 



5. As organs of circulation. — From the fact of transpira- 

 tion follows the next, that the leaves tend to produce a flow of 

 sap upwards. As evaporation takes place there is a continual 

 flow of water up the wood to take its place : the greater the 

 evaporation the more rapid the sap circulation. (See Chapter 

 XII.) 



The foregoing functions are performed by all leaves ; the 

 following, however, are exceptional. 



