586 ECOLOGY 



habits are related to external conditions may be inferred from many 

 trees and shrubs (e.g. poison ivy, Virginia creeper, various oaks) which 

 shed their leaves in regions of cold winters, but retain them in warmer 

 climates; furthermore, various plants (as the grape and the peach) be- 

 come evergreen in uniform tropical climates, and even those species that 

 remain deciduous (as the persimmon and the mulberry) have much 

 longer periods of leafage. 



The exact factors involved in leaf fall, that is, in the development of 

 the absciss layer, are imperfectly known. In the monsoon forest and 

 in other regions of periodic drought, it is probable that leaf fall results 

 directly from the desiccation incident to the increased transpiration 

 and decreased absorption during the dry period. Autumnal leaf fall 

 in cool climates probably is due to desiccation resulting from continued 

 transpiration at a time when absorption is diminished by reason of low 

 temperature, although desiccation due to dryness in the soil or air may 

 cause the absciss layer to develop in early summer. A severe frost 

 in early autumn may retard leaf fall through injury to the tissues that 

 develop the absciss layer. 



Leaf fall may result also from protracted wet weather, or from the transference of 

 a plant from a dry house to a moist chamber ; possibly the reduction of transpira- 

 tion if accompanied by strong turgor pressure may result here in the injection of 

 air spaces, and hence in impaired gas exchange and death. Early leaf fall some- 

 times is induced by diminished light (as in the lower leaves of tall herbs or of forest 

 trees) and by the attacks of parasitic plants and animals. In most cases leaf fall 

 seems to be associated with some impairment of activity, but why such impairment 

 ' should stimulate the development of a separation layer is not clear. The cause of 

 leaf fall in evergreens is as yet scarcely to be conjectured, there being little or no 

 obvious relation to external factors, except when the old leaves are in a sense 

 pushed off by growing shoots. Perhaps leaf activity gradually becomes impaired 

 through the continued accumulation of excreta and the increased clogging of the 

 stomata by dust, or perhaps such leaf fall is governed by internal factors. 



The advantages of leaf fall and of the evergreen habit. — The shedding 

 of leaves at the inception of a cool or dry period is of inestimable advan- 

 tage, especially in trees with delicate leaves, because of the enormously 

 reduced transpiration thus resulting. The leafless tree is one of the 

 most perfectly protected of plant structures, since impervious bud 

 scales and bark cover all exposed portions. So close is the relation be- 

 tween leaf texture and leaf fall that in temperate or in cold climates 

 one almost may determine by the feel of a leaf whether it is deciduous 

 or evergreen. While evergreens are more subject to winter transpiration 



