RELATION TO ENVIRONMENT 



527 



the smooth waxy coating of the stems of other plants e.g. Bloodroot 

 (Sanguinaria), Dicentra, etc. may be useful in preventing the ascent 

 of creeping insects. Within the flower, also, there may be densely 

 set hairs, which are impenetrable by the smaller insects that would 

 rob the flower of honey but not assist in pollination. The dense 

 felted masses of hairs at the base of the stamens in Cobcea scandens 

 are examples of this, and the similar hairs upon the stamen-filaments 

 in Tradescantia and other flowers are probably of the same nature. 



Protection of Xerophytes. The great development of thorns, 

 spines, and bristles, especially in plants of dry regions, is largely 

 protective, as these plants are especially liable to injury from hungry 

 herbivorous animals. The dagger-leaves of the desert Yuccas and 

 Agaves, and the terrible spines of the Cacti, are admirable examples 

 of the efficiency of such protective structures, and the thistles and 

 brambles of roadsides illustrate the same thing on a smaller scale. 



Odors. The strong odors and the poisonous or at least dis- 

 tasteful latex of many plants are probably also protective in their 

 nature. 



Myrmecophilism 



Among the most remarkable instances of symbiosis between plants 

 and animals is Myrmecophilism, or the association of ants with 

 various plants for protection, or otherwise. The frequent presence 

 of nectar glands upon leaves 

 and stems, e.g. those on the 

 petioles of species of Popu- 

 lus, Cherry, Kicinus, etc., is 

 usually associated with the 

 visits of ants, which protect 

 the plants from the attacks 

 of injurious insects, or even 

 ward off the attacks of large 

 herbivorous animals, which 

 are attacked by the ants. 



Much more remarkable 

 are the various tropical myr- 

 mecophilous plants which 

 provide shelter for ants, in 

 return for protection from FIG. 493. Acacia sphserocephala. A myrme- 

 other ants, principally leaf- cophilous plant. Ants live in the enlarged 



onttin^ forms A mono- the hollow thorns ' * and feed upon the 8pecial 

 i j A ! n llg t food-bodies, /, developed upon the leaves, 



first described forms or (After NOLL.) 

 these plants were species 



of Cecropia, a tropical genus of American trees. These possess 

 hollow stems, within which ants live. There are also produced 



