THE FORMS OF LEAVES 51 



thicknesses of very thin-walled cells is formed during the develop- 

 ment of the leaf. At the time of leaf fall the walls of these cells 

 become softened and finally disintegrate. A gentle wind or the 

 weight of a film of dew is then enough to cause the leaf to fall. In 

 some trees, especially in some of the oaks, the absciss layer is very 

 imperfectly formed and the leaves therefore hang on during the 

 greater part of the winter. On the other hand, in some compound 

 leaves an absciss layer forms first at the base of each leaflet and 

 these drop off. Later the petioles also drop. 



The cause of the formation of an absciss layer is not knowTi. In 

 regions with periodical dry seasons it is probably largely desiccation 

 brought about by increased transpiration without any increase in 

 rate of absorption. In regions with cold seasons, too, the cause may 

 be desiccation since, for plants, a cold season is a dry season, that is, 

 it is a season when transpiration continues but absorption is very 

 greatly reduced because of the low temperature of the soil. Con- 

 trary to a rather common belief frost does not ordinarily cause leaf 

 fall. In fact an early frost may delay leaf fall by interfering with the 

 development of the absciss layer. 



31. The Forms of Leaves.— Leaf form is to a large extent a matter 

 of inheritance. So true is this that very many species of plants are 

 easily identified by their leaves alone. To be sure there may be con- 

 siderable variation in the leaves of an individual plant. The first 

 leaves, or cotyledons, are always very different from the later ones. 

 In the garden bean the second pair of leaves as well as the cotyledons 

 are simple while the later ones are compound. A single mulberry 

 tree may show all variations from an unlobed leaf to one that is very 

 much lobed. The leaves of a single plant may also vary as much as 

 100 per cent or more in size. But with all this variation a mulberry 

 leaf never looks like an elm leaf. Inheritance is too potent a factor 

 for that. 



Nevertheless, there are some variations of leaf form that are more 

 or less characteristic of certain types of environment. Plants that 

 grow where there is an adequate supply of water have relatively 

 thin and broad leaves, while plants growing where there is difficulty 

 in getting sufficient water have smaller and thicker leaves. Further- 

 more, it has been shown b>- experiuKMit in the case of many species 

 that the same individual will develop larger and thinner leaves when 

 grown in a moist environment than when grown in a dry environ- 

 ment. These facts seem to indicate that here again water, perhaps 



