STEMS 743 



the hairs of Polygonum amphibium (p. 574), has a determinative rather 

 than a formative influence, as in the case of spinescent branches. 

 Possibly many spines and prickles are obligate rather than facultative. 



Spines and "natural selection." — A curious though widely accepted view is 

 that spines have been evolved by " natural selection " through age-long " compe- 

 tition " between plants and grazing animals. It has been supposed that plants 

 happening to have spines have survived and had progeny, while many others with- 

 out such " weapons of defense " have failed. While the xerophytic theory of spines- 

 cence has good experimental proof, the selection hypothesis has almost none, the 

 sole argument in its favor being derived from overgrazed pastures, where thistles, 

 brambles, and hawthorns tend sometimes to increase their area in place of more 

 palatable plants. Many considerations weaken the force of this argument. For 

 example, spinescent plants are not necessarily unpalatable ; hawthorns are eaten 

 by cows, until they can no longer reach the foliage, the lower parts of the trees often 

 being cropped into fantastic shapes; it is reported also that in Arizona grazing 

 animals avoid a relatively thornless cactus (probably because of its flavor), while 

 greedily eating a thorny variety. Furthermore, most thorns are tender and more 

 or less edible when young, that is, when the plants most need protection. 



As a whole, the plants most eaten by grazing animals are grasses, and it is well 

 known that grasses flourish as a result of grazing ; indeed, one of the prominent theo- 

 ries attempting to account for the prairie is that grazing animals keep down shrubs and 

 trees. Grasses increase and trees decrease in pastures in the Central States, because 

 the former have underground propagative organs and the latter not, spinescence 

 being a negligible factor. In the Eastern States shrubs and trees, whether spines- 

 cent or not, rapidly invade pastures because of the favorable climate, in spite of the 

 combined influence of rhizomatous herbs, farmers, and grazing animals. How- 

 ever, there are still stronger arguments against the selection theory. In the first 

 place, the close confinement of grazing animals is a recent and insignificant thing in 

 biological history ; the overplus of vegetation always has been so enormous that the 

 influence of herbivorous animals on plant evolution must have been infinitesimal. 

 In the second place, the culmination of spinescence is in deserts, where grazing ani- 

 mals are scarce, and where " competition," if such exists, is least important. Finally, 

 in so far as " natural selection " is applicable to the problem of spinescence, its in- 

 fluence has to do with the survival of spinescent plants rather than with the origin of 

 their spines. 



The advantages of spines. — Probably spines and prickles are of some 

 advantage as a means of protection from herbivorous animals, though 

 this advantage has been greatly overestimated. The perpetuation of 

 thistles in sheep pastures doubtless is due in part to leaf spinescence, and 

 the thorniest cacti (fig. 1063) certainly are amply protected from the in- 

 cursions of ordinary grazing animals. In the holly the upper leaves, 

 which are out of the reach of grazing animals, are less spinescent than 

 are the lower leaves. In the cacti, spines often are abundant enough to 



