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 {i.e. 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. — 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 development of spi- 

 nescent emergences, as in the roses, 

 there might seem to be agreement with 

 Ulex, but there is the fundamental 

 difference that here desiccation causes 

 the appearance of a new organ rather 

 than the reduction of an organ com- 

 monly better developed; in other words, 

 the influence of an increased supply of 

 water is directly inhibitory. Thus desic- 

 cation in the case of prickles and of 

 other spiny emergences, much as with 



Figs. 1066-1068. ■ 

 prickle development on the stem of 

 a wild rose {Rosa blanda) : 1066, a 

 portion of a young root sucker of 

 a mesophytic individual; note the 

 abundance of prickles; 1067, a 

 portion of a branch from the same 

 stem, showing an entire absence of 

 prickles; 1068, a portion of a 

 branch from a xerophyticindividual, 

 corresponding to that figured in 

 1067 ; note the numerous prickles. 



