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 honey-locust (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 

 xerophytic than in mesophytic 

 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- 



FIGS. 1064, 1065. Spinescence in Ulex euro- 

 paeus : 1064, an individual grown in saturated air; 

 note the conspicuous leaves; 1065, an individual 

 grown in dry air; note that the branches are re- 

 duced to spines. From LOTHELIER. 



