FUNCTIONS OF LEAVES 159 



caused by each of the guard-cells bending into a more 

 kidney-like form than usual, and the closing by a straight- 

 ening out of the guard-cells. The under side of the leaf, 

 free from palisade-cells, abounding in intercellular spaces, 

 and pretty well protected from becoming covered with 

 rain or dew, is especially adapted for the working of the 

 stomata, and accordingly we usually find them in much 

 greater numbers on the lower surface. On the other 

 hand, the little flowerless plants known as liverworts, 

 which lie prostrate on the ground, have their stomata on 

 the upper surface, and so do the leaves of pond lilies, 

 which lie flat on the water. In those leaves which stand 

 with their edges nearly vertical, the stomata are dis- 

 tributed somewhat equally on both surfaces. Stomata 

 occur in the epidermis of young stems, being replaced 

 later by the lenticels. Those plants which, like the 

 cacti, have no ordinary leaves, transpire through the 

 stomata scattered over their general surfaces. 



The health of the plant depends largely on the proper 

 working condition of the stomata, and one reason why 

 plants in cities often fail to thrive is that the stomata 

 become choked with dust and soot. In some plants, as 

 the oleander, provision is made for the exclusion of dust 

 by a fringe of hairs about the opening of each stoma. If 

 the stomata were to become filled with water, their activ- 

 ity would cease until they were freed from it; hence 

 many plants have their leaves, especially the under sur- 

 faces, protected by a coating of wax which sheds water. 



171. Measurement of Transpiration. We have already 

 proved that water is lost by the leaves, but it is worth 

 while to perform a careful experiment to reduce our 



