. A textbook of botany for colleges and universities ... Botany. 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 fo
. A textbook of botany for colleges and universities ... Botany. 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 Figs. 885, 886. — Leaf variation in the barberry (Ber- beris vulgaris); 885, a young shoot, whose lower bud (&) is sub
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
