. A textbook of botany for colleges and universities ... Botany. STEMS 741 So severe are the conditions at times that only those parts covered by the snow are able to survive. Spinescence. — The structural features of spines.—Stem spines are of two fundamentally different sorts, namely, reduced branches, as in the (Gleditsia), wild crab {Pyrus coronaria, fig. 1061), Prunus, and Crataegus (fig. 1062), and stem emergences, as in the roses (figs. 1066, 1068) and the gooseberries. In the spinescent branches, which often are compound, the branch character usually is easily recog- nize
. A textbook of botany for colleges and universities ... Botany. STEMS 741 So severe are the conditions at times that only those parts covered by the snow are able to survive. Spinescence. — The structural features of spines.—Stem spines are of two fundamentally different sorts, namely, reduced branches, as in the (Gleditsia), wild crab {Pyrus coronaria, fig. 1061), Prunus, and Crataegus (fig. 1062), and stem emergences, as in the roses (figs. 1066, 1068) and the gooseberries. In the spinescent branches, which often are compound, the branch character usually is easily recog- nized in youth through the presence of leaves and axillary buds, and scars may be found even on old spinescent branches. Spinescent emergences, however, commonly are simple, and may be either stout thorns or delicate prickles; both kinds occur in the roses, the former near the leaves, and the latter scattered along the stem. Perhaps the culmination of spinescence is seen in cacti (figs. 1063, 1040-1042), where all gradations occur between the stoutest thorns and the most delicate prickles. The factors determining spinescent branches. — Experiments on Ulex, Berberis (figs. 885, 886), and other plants show that spinescence may be induced by intense light and especially by desic- cation ; in Ulex the shoots de- veloped in moist air bear foliage leaves, while in dry air the branches and even the leaves become spinescent (figs. 1064, 1065). Pyrus coronaria and Prunus ameri- cana are much thornier in xerophy tic than in mesophy tic situations; Celtis occidentalis (the hackberry) is a spineless mesophytic tree, while Celtis occidentalis pumila is a thorny xerophytic shrub. Thus such spinescence seems to be a re- sult of hard conditions, the branches remaining reduced because of pronounced desiccation, supplemented, perhaps, by other fac- tors. Individuals of Ulex grown in concentrated glucose solutions de-. 1065 \\ Figs. 1064, 1065. — Spinescence in Ulex euro- paeus: 1064, an indi
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
