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 
