4-16 PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



The contrivance for protecting the green tissues of the cactuses, depending 

 upon a division of labour, is accomplished in a very poculiiir way. Our conception 

 of a plant is a stiff grey or brown stem bearing soft green leaves. In the 

 cactus-like plants, however, the most important types of which we have already 

 recognized in the Cactaceas of the New World, and the columnar Euphorbiaceae 

 of Southern Asia and Africa, everything is reversed. Here the stem is green 

 and succulent, and the leaves it supports are transformed into stiff grey or 

 brown spines. Food is conducted to the green transpiring tissue in the cortex 

 of the stem, in which, and not in the leaves, new organic materials ai'e produced. 

 The leaves which have been changed into spines, on the other hand, have to 

 keep guard that the green tissue in the cortex of the columnar or flattened 

 stem is touched no more than is necessary. This reversed state of things strikes 

 us as most strange in the opuntias (Plate IV.), because here the portions of the 

 stem have the form of thick elliptical leaves, and consequently are usually 

 held by non-botanists to be leaves. But the spines, or, sti-ictly speaking, the 

 leaves ti'ansformed into spines, occasionally attain to an extraordinary length 

 in these opuntias. They are 3-5 cm. long in Opuntia Tuna, decumana, and 

 magacantha, and even 8 cm. in Opuntia longispina. It has already been 

 mentioned that the buds of opuntias are based with very small barbed bristles, 

 and consequently these plants are armed with a twofold defence against possible 

 attacks with large spines, visible from afar, and with these horrible small incon- 

 fipicuous barbed bristles. In the cactus-like plants the variety of weapons is 

 very great, and if all the various forms of long and short, thick and thin, knotty 

 and smooth, straight-pointed and barbed, arched and wavy spines and bristles 

 were placed together side by side, quite a goodly collection of arms would be the 

 result. A single species often bears three or four kinds of weapons, and these 

 are arranged and distributed in a great variety of ways, and in this respect a 

 diversity is developed which has a fascinating effect on anyone who has an inborn 

 taste for such changes of form, and we can understand how it is that so many 

 lovers of flowers have devoted themselves to the study and culture of these 

 curious representatives of the vegetable kingdom. Although it is impossible 

 to show the connection between the kind of armour and the attacks to be 

 warded off in each individual instance, even the most cursory glance shows us 

 that the points of the spines, however these may be shaped and arranged, are 

 always placed in front of that portion of the stem which is best furnished with 

 green tissue. In the columnar euphorbias, e.g. in Euphorbia coirulescens, the 

 stems are furnished with shallow longitudinal grooves clothed with green tissue. 

 On the ridges between the grooves are inserted pairs of divergent spines with 

 their points in front of the grooves, and thus ward off every assault on the green 

 tissue. Exactly the same thing is seen in the columnar Cereus, and in the cone- 

 shaped Echinocactus and Melocactiis. 



On looking at these columnar, flattened and spherical cactuses, the question 

 arises whether it is necessary for them to be surrounded with such a complicated 



