260 EVOLUTION OF PLANTS 
not render the plants entirely immune, is shown, how- 
ever, by the attacks of certain animals, especially insects, 
which have apparently adapted themselves to these 
peculiar conditions. Nevertheless, there is no question 
that such plants suffer very much less from animals 
than they would if these means of protection were 
absent. It has been thought that the sharp needle-like 
erystals or rhaphides, which occur so abundantly in 
many Monocotyledons, may deter animals from eating 
them, as many of them, especially the aroids, have an 
excessively acrid taste, which is supposed to be due 
to the mechanical irritation produced by these sharp 
crystals. 
The presence of spines, thorns, and prickles, as well 
as rough hairs upon the stems and leaves, is doubtless 
mainly protective. They are usually most noticeable 
in plants of dry regions, where the scanty vegetation is 
peculiarly exposed to the attacks of herbivorous animals. 
The cacti are very perfect instances of this peculiarity. 
The terribly sharp thorns of these plants render them 
perfectly safe against the attacks of hungry animals, 
which eat them greedily if care is taken to remove 
their thorny armor. Where desert plants are not 
thorny, they ‘are usually ill-scented and thus distaste- 
ful to herbivorous animals, 
But one more of the most remarkable cases of recip- 
rocal relations between plants and animals will be cited, 
namely, the peculiar arrangement known as myrmecoph- 
ily, where ants inhabit certain parts of trees, to which, 
in return for shelter, and sometimes food in the form 
of honey-like secretions or peculiar albuminous fatty 
bodies, they protect the plants from the ravages of other 
