. A textbook of botany for colleges and universities ... Botany. 742 ECOLOGY velop spines even in moist air. A recent worker claims that the leafy shoots of Ulex usually developed in moist air are merely juvenile shoots. Spinescetit branches, as appears from the preceding, are dwarfed shoots, and are produced much as are other dwarfed stems. However, there is no tendency toward lateral enlargement, as in most dwarfed stems, but rather the reverse, since one of the chief characteristics of spines is attenuation. If elongation occurs when the growth conditions are very favorable, and lateral enl
. A textbook of botany for colleges and universities ... Botany. 742 ECOLOGY velop spines even in moist air. A recent worker claims that the leafy shoots of Ulex usually developed in moist air are merely juvenile shoots. Spinescetit branches, as appears from the preceding, are dwarfed shoots, and are produced much as are other dwarfed stems. However, there is no tendency toward lateral enlargement, as in most dwarfed stems, but rather the reverse, since one of the chief characteristics of spines is attenuation. If elongation occurs when the growth conditions are very favorable, and lateral enlargement accompanied by dwarfing when they are less favorable, then attenuation with dwarfing { spines- cence) may be a result of very unfavorable conditions. Attenuation seems to imply progressive severity in the developmental conditions, the supply of structural ma- terial becoming less and less as the branch develops. Furthermore, there is an in- creasing development of mechanical tissue in proportion to the other tissues, the tip being composed chiefly of thick-walled elements, which account for its extreme stiffness; this fact also fits in with the desiccation theory of spinescence. Desic- cation does not account readily for all spinescent branches, since Crataegus and Gleditsia develop them profusely in moist and fertile flood plains; furthermore, thorny lianas are extremely abundant in humid tropical forests. The factors determining spinescent emergences. â The cause of spines- cent emergences is much more difficult to determine than that of spines- cent branches. Their variation is considerably less and their develop- ment is not so obviously related to severe conditions. The stem of Rosa blanda frequently is smooth, but in xerophytic conditions, prickles appear in abundance (figs. 1066-1068): oddly enough, abundant prickles also char- acterize vigorous rose and gooseberry suckers in mesophytic habitats. So far as desiccation or other hard con- ditions favor the de
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
