124 THE EVOLUTION THEORY 



beautiful flower-spikes with the thick, soft felt leaves, which grow 

 on stony or sandy soil. Harmless as they look, they are much 

 disliked by animals as food, for the thick hairy felt which covers 

 them breaks up in the mouth, and sticks in the folds of the mucous 

 membrane, causing burning sensations and other discomforts. They, 

 too, are therefore spared by grazing animals, but they have smaller 

 enemies, like the caterpillars of the genus CucuUia, which, however, 

 never completely destroy them, but only eat large holes in their leaves. 

 Let us now consider in somewhat greater detail the true thorns, 

 the most conspicuous protection of many plants. It is very remarkable 

 that these are always so placed, and so regulated as to their length and 

 character, as to afford protection to the most important and the most 

 exposed parts of the plant. Thus many bushes, which would other- 

 wise be in danger of being completely devoured by cattle, are stiff 

 with thorns which are nothing else than pointed, 

 hard twigs without, or with very little foliage. 

 Among these are the sloes, the buckthorn 

 (ElMmnus), the sea-buckthorn (Hippophde), and 

 the barberry (Berheris). In the last-named 

 three thorns arise in a group, and protect 

 the young bud from danger in three directions 



(Fig. 21). 



The fine-leaved mimosas of the tropics have 

 similar but very long and sharp thorns, and 

 . ?^°- /I; , -^ P^^'if °^ ? their leaves are movable and sensitive, so that, 



twig 01 Barberry (Beroens ' ' 



vulgaris) in spring; after when they are touched, they shut up and draw 

 ®'^"^'^' back behind the rampart of stiff thorns, which 



are just of the right length to protect them. 



In many thorny bushes only the young shoots of each spring 

 remain green through the summer, and in autumn they become 

 transformed into thorns, under whose protection the shoots of the 

 following spring will develop. Sometimes, too, the leaf-stalks 

 are modified in the course of the summer into thorns, as in 

 Tragacanth {Astragalus tragacantha). In this case the young leaves 

 are protected by a circle of thorns, consisting of the leaf -stalks of the 

 preceding year which have not fallen off (Fig. aa, A, B, G). 



I should have to go on for a long time with my exposition, even 

 if I were to confine attention to the essential facts ; we shall, there- 

 fore, only recall the well-known phenomenon of the Cactuses, in 

 which the leaves are entirely transformed into spines, which may 

 attain a length of eight centimetres, while the fleshy stem alone 

 represents the green — that is, the assimilating parts of the plant. 



