LEAVES AND THEIR USES 255 



Not only are whole leaves separated in this way from the 

 plant that bears them ; in the case of some divided leaves 

 like those of the Virginia creeper or the horse-chestnut 

 (Fig. 149), each individual leaflet is separated in the same 

 way from the main stalk of the leaf. The fall of leaves may 

 occur not only in the autumn, but at any time of the year 

 when for some reason, such as lack of water, a part of the 

 plant dies. So the dropping of leaves may be observed at 

 almost any time on many common house and garden plants. 

 The leaves of the pine, which live for several years, are cut 

 off in the same way when they finally die. This method of 

 bringing about leaf -fall is found, generally speaking, only in 

 dicotyledons and gymnosperms. The leaves of most mono- 

 cotyledons (such as the Indian corn and the lily), and of 

 some dicotyledons like the oak, do not form a separation 

 layer. As a result, although they may die and wither, 

 they do not fall at once, but remain attached until they 

 decay or are torn away. 



266. How Do the Death and Fall of Leaves Help the 

 Plant ? The answer to this question, as to so many others, 

 lies in the plant's relation to water. We know that plants 

 in those regions which have cold winters can obtain com- 

 paratively little water from the soil during the winter. There- 

 fore, unless the area from which evaporation may take place 

 is greatly reduced, the plant will be injured or killed by the 

 too rapid loss of water which it cannot replace. Most of the 

 plants of temperate regions that live over the winter (that 

 is, biennials and perennials) avoid this danger by shedding 

 their leaves, from which evaporation chiefly occurs, in the 

 way we have just observed. The cork layer which covers 

 the scar left on the stem or branch by the falling of the leaf 

 serves to prevent evaporation at that point ; it is also a 

 protection against infection by the spores of parasitic fungi. 

 Since most of the monocotyledons of temperate countries 

 are either annuals, or perennials whose above-ground parts 



