6o4 



ECOLOGY 



ably suffice in large part to explain such cases.' In Thuja (fig. S74) the first leaves 

 are needle-shaped, as in the juniper, but after a year or two there appear flattened 

 lateral branches with appressed awl-shaped leaves (fig. 875). Thenceforth awl- 

 shaped leaves continue to develop through life, but needle leaves again develop 

 if the plant is placed in a moist chamber. Whether this represents a direct re- 

 action to the new conditions or merely " rejuvenescence " is not known. Very strik- 

 ing changes in form are exhibited by Eucalyptus globulus, which has a thinnish, 

 horizontal, ovate shade leaf and a thick, vertical, lanceolate-falcate sun leaf, the 

 differences appearing to be due largely to differences in transpiration. 



In Geum virginianum and Ranunculus aboriivus, and in many similar 

 plants, the leaf changes are more complicated than in Campanula, 



the basal rosettes of 

 roundish winter leaves 

 being succeeded by 

 variously divided stem 

 leaves; in Geum the 

 round leaves are fol- 

 lowed by pinnate and 

 later by three-parted 

 leaves (figs. 876-879). 

 In Silphium lacinia- 

 tum, narrow, undi- 

 vided early leaves are 

 followed by broader, 

 much-lobed adult 

 leaves. In various 

 oaks not only are the 

 early leaves relatively 

 entire and the later 

 leaves lobed, as in 

 Geum, but the upper 

 leaves are more lobed 

 than the lower, what- 

 ever their time of appearance, while leaves on vigorous young shoots 

 (such as stump suckers) are scarcely lobed at all (figs. 880, 881). 

 Perhaps the lobation of the upper leaves, as well as their smaller 

 size and greater thickness, is due to the increased transpiration to 

 which they are subject. The large leaves of vigorous suckers also 



» In some plants, as the Cottonwood, the upper leaves are larger than the lower, ap- 

 pearing to be better nourished. 



Figs. 885, 886. — Leaf variation in the barberry (Ber- 

 beris vulgaris) ; 885, a young shoot, whose lower bud (&) 

 is subtended by an ordinary foliage leaf (/), while the 

 upper (b') is subtended by a spine (s) ; the leaf and spine 

 are homologous (i.e. similar in position) and develop from 

 similar primordia); 886, a bud similar to b' in 885 but 

 somewhat older. 



