PROTECTIVE ADAPTATIONS IN PLANTS 121 



plants, as a peculiarity designed for the use and profit of man. But 

 these ethereal substances are obviously a means of protection against 

 the depredations of seed-eating birds, for a sparrow which was allowed 

 to eat three or four seeds of cummin died very soon afterwards. 



Many plants produce bitter substances in their green parts, and 

 so secure at least some measure of protection, as is the case with the 

 majority of mosses, the ferns, and species of Plantago and Liiuiria. 

 Others, again, deposit silicic acid in their cell-walls, or develop in 

 addition a very thick epidermis, so that they afford at the best an 

 unpleasant food, e.g. many grasses, the horse-tails, the rhododendron, 

 and the bilberry. Others, again (AlcJiemilla vulgaris), have cup-shaped 

 leaves, which retain rain and dew for a long time, and this protects 

 them from grazing animals, which are unwilling to touch wet grass 

 and plants. 



Especially widely distributed and diverse is the protection of 

 plants by sharp thorns and spines. It it extremely interesting to 

 note in how many different and advantageous ways this armature 

 is disposed. 



Obvious at once is the fact that thorns and spines only occur on 

 those parts which are naturally exposed to attack. Thus we find 

 them particularly strong in young plants, and on the lower parts of 

 older ones. The holly, for instance, has crenate, spinose leaves only 

 to the height to which grazing animals can reach ; beyond that the 

 leaves are smooth-edged and spineless, like those of the camelia. It 

 is almost the same with some wild pear-trees, which are quite covered 

 with thorns as long as they are low, but afterwards grow a thorn- 

 less crown. 



Similarly, low bushes, when they are armed with thorns or the 

 like at all, are covered with them all over, like the rose-bush. 



When the leaves of a plant are spinose the spines are disposed on 

 the parts usually attacked ; and thus we understand why the 

 enormous floating leaves of Victoria regla should have on their 

 under surface long, pointed spines which, especially at the upturned 

 margin, attain a length of several inches ; it is from water animals — 

 water snails — that danger threatens them. 



Thorns are developed in the most diverse ways. In many of the 

 bushes on the coast of the Mediterranean true leaves are wanting 

 altogether, the green branches and twigs being themselves the 

 assimilating parts, and these are so stiff and rigid, so like some 

 kind of thorn, that they suffice to scare off any greedy herbivore. 

 Among our own bushes the Broom (Spartium scoparium) may be 

 taken as an example of this class. 

 I. i 



