STRUCTURAL DEFENCES OF PLANTS AGAINST SNAILS. 
287 
comiectioii between a definite group of animals and tlie protective 
arrangements of certain plants, yet such reflex contrivances un- 
doubtedly exist, although the same protective devices may be 
operative against very diverse animals. Certain groups of plants are, 
however, distinguished liy the predominance of a certain mode of 
protection, although few liave the same protection in all their parts, 
dowers having usually different defensive devices to the vegetative 
portions, and the internal protection when present differs in character 
from that of the exterior. 
The Structural Defences embrace a prickly or hairy investment, 
and the hardening of the stems and foliage by the silicidcation or 
calcidcation of the more exposed tissues, and such plants are probably 
still acceptable to molluscan palates when their external defences can 
be overcome. 
The Silicidcation of tlie exterior of many of our gxasses. Horsetails 
(E(|uisetacem) and Cyperacem not only gives rigidity and dimness to 
the plants, but is a real protection against snails, heliciue teeth 
being practically helj)less against the defence it od’ers; its efdciency is 
shown in some tropical grasses, in which this silicidcation is so pro- 
nounced as to render them totally undt even for feeding cattle. 
Mosses are also protected by their highly silicided tissues, shown 
in the stid' pointed hairs, serrate margins and rough capsule stems, 
features particulai'ly noticeable in Fiinaria hygrometrica. 
Calcidcation of the tissues is the defence of many plants, and this 
protection may be internal and due to the deposition of carbonate of 
lime within the cells. Erysimum cheiranthoides being rendered so 
hard from this cause that it is avoided by even Agriolimax agrestis, 
whereas Chara and other aquatic groups are calcided externally, 
assuming an e(|ually protective incrustation of the same substance. 
Oxalate of lime is abundant in the outer tissues of Orchids, 
Amarjdlids, Narcissi, Tyylm latifoUa, Arum maculatum and other 
plants, and generally exists in the form of Raphides a needle), 
minute, needle-like crystals, 2Uo 'jf “icli in length, which 
are always most plentiful near the surface and confer comparative 
immunity against attacks by snails and many other animals, as they 
wound the palate and cau.se a .strong burning sen.sation within the 
mouth of any creature attempting to feed upon the plants containing 
them ; they have also been known to give rise by contact to a 
form of eczema, even in human subjects. 
