PROTECTION OF GREEN LEAVES AGAINST ATTACKS OF ANIMALS. 433 



terminology they are known as spines and prickles. A structure which is mainly 

 composed of wood, or whose interior is at least traversed by vascular bundles 

 springing from the wood, and which, therefore, ends in a firm sharp point, is called 

 a spine (spina). On the other hand, a prickle (aculeus) is a structure which 

 proceeds from the epidermis or cortex of a plant member, contains no vascular 

 bundle within, may be multi- or unicellular, but always terminates in a point which 

 is capable of wounding the skin of the offender. This distinction is not always an 

 easy one to make, and botanists have never laid much stress upon it. 



Spines and prickles may arise from all the plant members and organs, and 

 appear at all heights. They are observed most usually on or near the green tissues 

 to be protected, but often even the road to the green organs, passing over the leaf- 

 stalk, the stem, and occasionally also over aerial roots, is provided with prickles 

 and spines in order that in this way the animals which feed on vegetables, 

 particularly snails, which creep up from below, may be kept off. Thus very 

 pronounced spines are seen, for example, on the aerial roots springing from the 

 lower part of the stem in Trithrinax aculeata. The lower portions of the main 

 axes up which these animals must climb in order to reach the green portions are 

 armed with spines or prickles, in many Bomhax and Pandanus, in Erythrynese, 

 gleditschias and roses, and in the fan -palms very abundantly on the leaf- 

 stalks. 



The size, direction, position, and distribution of the weapons depends generally 

 upon the nature of the attack, on the form and size of the food-seeking animals. 

 and on the nature of the implements at their disposal The gigantic floating leaves 

 of the Victoria regia are only armed with prickles on the under surface and on 

 the tumed-up margin, i.e. only where they are exposed to the attacks of plant- 

 eating aquatic animals. It is also an interesting fact that many woody plants are 

 only protected when young, i.e. while they are short and their foliage can be 

 reached by ruminants, viz. by goats, sheep, and oxen; but on the boughs and 

 branches removed beyond the reach of the mouths of these animals, no prickles 

 and spines are developed. Young, low trees of the Wild Pear, only one or two 

 metres in height, bristle with the spines into which the ends of the woody branches 

 are transformed; while the branches of the crown of trees four or five metres 

 high remain without spines. The same thing occurs in the Chinese Gleditschia 

 (Gleditschia Chinensis), and in the Holly (Ilex Aquifolium). In the latter it 

 can be seen that the leaves of the crown of tall trees have almost entire margins 

 and are unarmed, while the margin of the leaves in shrubby specimens is drawn 

 out into bristle-like, pointed teeth. 



Plants armed with weapons for warding off the attacks of animals may be 

 arranged together in two groups. One of these consists of those forms which 

 protect their green tissue by structures actually developed on the organs in 

 question, and the other group comprises those forms which have no such capacity 

 of self-help, where, rather, one member protects another, and where division 

 of labour has brought it about that certain plant-organs deprived of chlorophyll 



VOL. L 28 



