114 - ELEMENTS OF BOTANY. 
passive way, as so many mere holes in the epidermis might, 
but to some extent they regulate the rapidity of transpiration, 
opening more widely in damp weather and closing in dry 
weather. The opening is produced by each of the guard-cells 
bending into a more kidney-lke form than usual, and the 
closing by a straightening out of the guard-cells. The under 
side of the leaf, free from palisade cells and abounding in 
intercellular spaces, is especially adapted for the working of 
the stomata, and accordingly we find them in much greater 
numbers on the lower than the upper surface. On the other 
hand, the little flowerless plants known as liverworts, which 
he 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 distributed somewhat equally 
on both surfaces. Stomata occur on 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 working 
and proper 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 activity would cease until 
they were freed from it; hence many plants have their leaves, 
especially the under surfaces, protected by a coating of wax 
which sheds water. 
144. Experiment 26.* Amount of Water lost by Transpira- 
tion. — Procure a thrifty hydrangea! and a small ‘‘ india-rubber plant,’’ 2 
each growing in a small flower-pot, and with the number of square inches - 
1 The common species of the greenhouses, Hydrangea hortensis. 
2 This is really a fig, Ficus elastica. 
