288 
ClIEMirAL DEFENCES OF PLANTS AGAINST SNAILS. 
Tlie Chemical Defences are constituted by more or less nauseous 
secretions, most abundant in the parts most liable to attack and due 
to bitter principles, tannin, acrid juices or exudations, essential and 
volatile oils, alkaloid, mucoid, or gelatinous secretions and by other 
obnoxious substances as yet undetermined. 
Tannin or 'rannic .Vcid exists in the epidermal cells and hairs of 
ferns, ro.ses, geraniums, ericas, pajnlionaceous and leguminous plants, 
and is a very powerful protective ; many compositie are also excep- 
tionally rich in this substance, to which they owe their disagreeable 
smell, while to V((Hsneri((, H jidrocharh, PotdmiHjcton and other 
a([uatic plants it gives j)artial protection, supplemented in many cases 
by the dei)Osit of nnmerons rai)bide.s ; while in the freshwater alg:e 
belonging to the genera Vaucherhi, >Sj>iro(ji/r((, and in Conferva), etc., 
it exists in such ([uantities that a good ink can be jwepared from the 
alcoholic solution of their chloroi)byll. 
.Vcrid juices are repugnant to snails and therefore the sap of Iiunu'.r, 
O.i’dl/.'i, etc., ethciently })rotects these plants from many snails. 
Essential oils also safeguard the plants containing them, the gallic oil 
of the Allium tribe, the essential oils of Jtufd and the Lahiata- con- 
ferring an immunity from attack by most .sj)ecies. The Willow-herbs 
receive a certain amount of protection from the acrid volatile oil 
.secreted by the glandular hairs, while the Alkaloids pre.sent in the 
Solanataao and the bitter principle characterizing the Centians perform 
the same duties to the plants secreting them. 
jMucoid and gelatinous secretions or excretions, when well developed, 
are also an elticient barrier against snails. Xitelld si/iicdrpd secretes 
a gelatinous investment which acts as a strong deterrent to Limniva 
sfd(jii((/ts and jirobably other sj)ecies, while (dlh'md (jranuxiim, a 
gelatinous lichen, is left untouched by both land and iluviatile snails. 
These various defences remb'r it jirobable that the mollusca though, 
api)arently, living in the midst of ])lenty are often in reality only able 
with diftlculty to eke out a lU'ccarious existence. This is confirmed 
by the exhaustive exi)eriments upon various snails carried out by 
JMr. (Jain, which demonstrateil, for e.xample, that out of 192 varieties 
of food — cbielly the commoner plants of the vicinity of Newark, 
Notts. — offered to a. colony of Helix hortenai^, no less than 139 were 
ai>parently so elliciently protected that they were i[uite untouched and 
17 others only slightly nibbled, even after iletinite periods of starva- 
tion. Of the .3(1 remaining foods more or less freely partaken of, 21 
