114 ELEMENTS OP 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-like 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 stbmata, 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 

 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 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," '^ 

 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. 



