438 PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 



soil of the elevated steppes, where they are so well protected by their needle-shaped 

 leaves, projecting all round from the stem, that they are never eaten by grazing 

 animals. 



With the acicular form of foliage-leaves are ranked those which may be best 

 compared to the process of the sword-fish. The outline of the leaves belonging 

 to this form is linear, or linear-lanceolate, generally they are elongated, and 

 often also slightly curved. Many of them are thickened and fleshy, but at the 

 same time very hard and rigid on the outside. The points, produced by the 

 terminations of the vascular bundles, spring from both margins of the leaf, and 

 in the majority of instances stand at right angles to the margins; more rarely 

 are they directed forwards. Each leaf either ends in a strong-pointed thorn, 

 as in the agaves, or in a bundle of threads, as in Bonapartea and Dasylirion. 

 The teeth on the leaves of the last-named plants remind one most in form, 

 surface, and colour, of the scales of a shark, and can inflict frightful wounds on 

 those who come too closely into contact with them. The table-land of Mexico 

 is particularly rich in plants with leaves armed in this manner; that country is 

 especially the habitat of agaves and Bromeliacese, of species of Dasylirion and 

 Bonapartea. The Cape also is the home of a series of these forms, viz. those 

 belonging to the genus Aloe. Species of Eryngium, with agave-like leaves 

 {Eryngium bromelicefolium, pandanifolium,, &c.) belong to Mexico and South 

 Brazil. It is worthy of note that several aquatic plants, such as Hydrilla, 

 Naias, and the Water Soldier (Stratiotes aloides), have their leaves similarly 

 armed, and are thus protected from the attacks of plant-eating aquatic animals. 



A third form of foliage-leaf, armed with spines, is that of the thistle. The 

 word thistle is here used in its widest sense, and is not restricted to species 

 of the genus Carduus and Cirsium (see fig. 115). By the term thistle-leaves are 

 indicated all those which are more or less lobed and divided, whilst the margins 

 and the extremities of the lobes are provided with stifif, projecting spines. Such 

 leaves are possessed by very many composites of the genera Carduus, Cirsium, 

 Chamoepieuce, Onopordon, Carlina, Echinops, Kentrophyllum, Carduncellus, 

 especially also in many Umbelliferee (e.g. Eryngium amethystinum, Echinophora 

 spinosa, Cachrys spinosa), some nightshades (e.g. SolanuTn argenteum, pyra- 

 canthos, rigescens), several Cycadese (Zamia, Encephalartos), and are very strongly 

 developed in Acanthus, of which a species. Acanthus spinosissimus, growing 

 in the Mediterranean floral district, is illustrated in fig. 116. 



Nowhere in the whole world is the thistle-leaf met with so abundantly and 

 in such manifold varieties as in the Mediterranean flora; Spain and Greece, 

 Crete and Algeria, are particularly rich in districts covered with thistles. Thistle- 

 leaves often appear divided into three, four, or five portions, and split up into 

 numerous points, sections, and lobes. The ends of all the separate portions 

 being transformed into stifi" points, not much remains of the green tissue of the 

 leaf; only a small narrow green lamina is seen, from which radiate out yellow 

 and white spines on all sides, like lances of various lengths 



