434 PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



and metamorphosed into weapons assume the protection of tlie unarmed adjoining 

 chlorophyll-bearing members. 



To the first division belong chiefly most of those leafless plants which have 

 developed green tissue in the cortex of their branches and twigs. Indeed, the 

 green branches of these plants are, as a rule, so firm and rigid that one would 

 imagine they would scarcely ever tempt animals to eat them. But " hunger is a 

 hard master", and in cases of necessity, as shown by experience, even the stiff 

 switch-like shrubs of the Mediterranean and other floral districts are attacked. In 

 order that they may not succumb entirely to these assaults, many of the leafless 

 green-branched plants are suitably armed by the possession of spines at the ends of 

 their green branches, which confront the assailants. Many of these plants, indeed, 

 are actually built up entirely of much-branched green spines, which fact, of course, 

 gives them a very peculiar appearance. The spinose flora of Spain and of the 

 opposite coast of Africa exhibits a whole series of these plants, but here only the 

 Furze {Ulex nanus, Gallii, micranthus), and the spring Asparagus {Asjmragua 

 horridus, Broussonetia, and retrofractus) need be cited as examples. Also the 

 green leaf -like branches of plants with flattened shoots, which are not protected by 

 poisonous substances like those of Phyllanthus, run out into sharp points, as may 

 be seen in the European Butcher's-broom (Rwscus aculeatus), illustrated in fig. 82, 

 and in the South American Colletia cruciata, represented in fig. 83 ^. 



The weapons developed on green leaves are far more complicated than the 

 implements with which green stems are furnished. In some instances points which 

 wound aggressors project from the ends of the ribs and veins which form the ground- 

 work of the leaves, rising up like needles above the green tissue of the foliage; in 

 other cases they consist of cells and groups of cells which originate from the 

 epidermis of the green leaf, and are inserted, sometimes at the margin, sometimes 

 on the surface, like little daggers. In the first instance the vascular bundles, 

 which are seen traversing the leaf as ribs, are provided, at the point where they 

 project beyond the green tissue and terminate as spines, with a covering of very 

 hard cells; in the latter case the cells and cell-groups springing from the epidermis 

 and rising up as prickles, bristles, and pointed hairs, exhibit thickened and strongly- 

 silicified walls. The following equipment appears particularly often in several 

 pines, many grasses, sedges, and rushes, in species of the genus Yucca, in several 

 caryophyllaceous plants (Drypis and Acanthophyllum), in Acantholimon, 

 belonging to the order PlumbaginesB, and in some saltworts and succulent plants 

 (Umbilicus spinosus, Sempervivum acuminatum). The green leaves are 

 numerous, usually crowded thickly together to form a tuft, and project from 

 the axis in all the directions of the compass; they are rigid, undivided, linear, 

 round or triangular in cross section, and terminate in a sharp, strong, piercing spine. 

 This form of leaf may be termed acicular (or needle-shaped). In many cases, at all 

 events, such leaves have exactly the form of needles, and in regions where the 

 unaltered products of nature are still preferably used as tools and utensils, they 

 actually serve as needles. That plants possessing leaves with these needle-like 



