DEFENCES UF PLANTS AGAINST SNAILS. 
280 
organs, although this method only allows a purely passive selection 
of the nutrient particles to be made. The more active Gastropoda, 
however, often exercise great discrimination in the selection of their 
food, althongh their tastes are not identical wiih onr own, except 
perhaps in their fondness fur saccharines, most .species being partial 
tu the sweet parts of j)la.nts. In other respects our tastes are different; 
C7ntroj>/ii//lnm temidum, though relished by many snails, produces 
unpleasant sen.sations on the human tongue, while many papilionaceous 
or pea-like i)lants, pleasant or merely insipid to ourselves, are carefully 
avoided by snails or only nibbled under stress of hunger. Although 
many Gastropods are more or less omnivorous, yet some species retain 
or have ac([uired a special power of feeding upon certain plants, which 
are adeiiuately defended against others not similarly modified. 
Plants, being the staple food of mollusks, would be much more 
■severely ravaged by them if their constant attacks during countless 
ages had not contributed to develop a variety of i>rotective devices, 
liruhahly most formidable in those plants which formerly suffered 
must severely from their depredations. These acipiired defences are 
now su universally })resent that Stahl has not found a single phanero- 
gam unfurnished with some means of pnffection, rendering such 
plants more or less unpalatable or difiicnlt of access, so that only 
dire necessity compels the omnivorants to feed sparingly upon the 
least protected parts of the less perfectly defended species, and 
although Arum atrr and AgrioUma.v agrestis are so pre-eminently 
omnivorous and greedy that few i)lants are altogether safe from their 
attacks, yet even their ravages are infinitely reduced in extent by the 
varicil ohstacles to he overcome before the desired food is obtainable. 
'I'liese harriers are indeed often so insuperable that certain sensi- 
tively organized species feed by preference upon dead or decaying leaves, 
jirohahly thus exhibiting their keen sn.sceptibility to the chemical pro- 
teciion so many j)lants enjoy, a jn'otection which, being dissipated on 
the fall of the leaf, enables them to he then partaken of without injury. 
Cultivated i)lants alone are not adeipiately armed, having pro- 
bably lost l>y .selective culture the re])ellant substances or structures 
to whicb they owed their i)reservation in a wild state, so that they are 
now greedily devoured l)y snails and slugs, and owe their continued 
existence in their present form solely to man’s protection. 
The Defensive Devices of i)lants operative against .snails may be 
mechanical or chemical, and although it is difficult to recognize a close 
