LEAVES 



585 



bananas (fig. 846); (2) evergreens with broad and stiff leaves, the so- 

 called broad-leaved sclera phylls, as the live oak and the holly (fig. 809) ; 

 (3) evergreens with stiff, needle-like leaves, as in the pine and the spruce 

 (fig- 955); (4) succulent desert evergreens, as Agave (fig. 921); and (5) 

 leafless evergreens, 

 such as the cacti (fig. 

 1035) and Ephedra. 

 A transition to de- 

 ciduous trees is seen 

 in the potential ever- 

 greens, such as Ilex 

 decidua, which, 

 though deciduous in 

 the northern states 

 (as suggested by the 

 specific name), is 

 evergreen farther 

 south, as are various 

 oaks that are decid- 

 uous in the north. 

 Magnolia grandi- 

 flora, a true ever- 

 green in the Gulf 

 states, retains its 

 leaves at its northern 

 limit, although they 

 die before the winter 

 is over, thus resem- 

 bling the beech and 

 those oaks in which 

 the autumnal absciss 

 layer is imperfectly 

 developed. 



The causes of leaf fall. — The leaf behavior of deciduous trees and 

 of tropical evergreens obviously is related to external factors, in the 

 former being associated with cKmatic periodicity (either of moisture, as 

 in the monsoon forests of India, or of temperature, as in the northern 

 deciduous forests), while in the latter it is associated with uniform 

 moisture and temperature. That the deciduous and the evergreen 



Fig. 846. — A group of banana trees (Musa) ; the gigantic 

 leaves have been much frayed by the wind (marginal me- 

 chanical tissue being poorly developed), giving the effect of 

 a pinnate palm leaf; the shrubs beneath the banana trees 

 are coffee plants (Coffea arabica) ; Xalapa, Mexico. — 

 Photograph supplied by Land. 



