LEAVES — FUNCTION OR WORK 99 



off by the leaves. The plant shows the effect (how?). 

 On a hot dry day, note how the leaves of corn " roll " tow- 

 ards afternoon. Note how fresh and vigorous the same 

 leaves appear early the following morning. Any injury to 

 the roots, such as a bruise, or exposure to heat, drought, or 

 cold may cause the plant to wilt. 



Water is forced up by root pressure or sap pressure. 

 (Exercise 99.) Some of the dew on the grass in the morn- 

 ing may be the water forced up by the roots ; some of it is 

 the condensed vapor of the air. 



The wilting of a plant is due to the loss of water from 

 the cells. The cell walls are soft, and collapse. A toy 

 balloon will not stand alone until it is inflated with air 

 or liquid. In the woody parts of the plant the cell walls 

 may be stiff enough to support themselves, even though 

 the cell is empty. Measure the contraction due to wilt- 

 ing and drying by tracing a fresh leaf on page of note- 

 book, and then tracing the same leaf after it has been 

 dried between papers. The softer the leaf, the greater 

 will be the contraction. 



Storage. — We have said that starch may be stored in 

 twigs to be used the following year. The very early flowers 

 on fruit trees, especially those that come before the leaves, 

 and those that come from bulbs, as crocuses and tulips, 

 are supported by the starch or other food that was organ- 

 ized the year before. Some plants have very special stor- 

 age reservoirs, as the potato, in this case being a thickened 

 stem although growing underground. (Why a thickened 

 stem? p. 84.) It is well to make the starch test on winter 

 twigs and on all kinds of thickened parts, as tubers and bulbs. 



Carnivorous Plants. — Certain plants capture insects and 

 other very small animals and utilize them to some extent 

 as food. Such are the sundew, that has on the leaves 



