METABOLIC PROCESSES IN THE PLANT. 345 



following kinds of leaves : Trifolium pratense (tannin-containing), 

 Rumex, Oxalis (acid), Ruta graveolens (rich in ethereal oil), 

 Ranunculus acris, and Tropseolum majus. In each crystallising 

 lOflass are placed with the snails one or two fresh leaves, but at 

 the same time we supply leaves from which the chemical protec- 

 tive material has been extracted by warming with alcohol, and 

 then drying in the sun or drying chamber, and finally washing 

 with distilled water. Since the snails feed chiefly in the evening 

 and at night, it is always advisable to put the glasses with their 

 contents aside for a good time. The fresh leaves are not at all 

 or only little attacked by the snails ; they rapidly destroy the 

 extracted leaves. The following experiment also definitely teaches 

 that, e.g., tannins and vegetable acids must afford plants an excel- 

 lent means of protection against snails. We supply the snails in 

 the crystallising glasses with thin slices of carrot (Daucus carota), 

 and at the same time with slices which have been killed by boiling 

 water, dried in the oven, and finally soaked in a 1 per cent, 

 solution of tannin, or a 1 per cent, solution of acid Potassium 

 oxalate. The fresh pieces of carrot are very readily eaten by the 

 snails ; the slices impregnated Avith tannin or acid they reject 

 entirely. 



We must not fail to make the following experiments in order 

 to acquaint ourselves with the action of mechanical protectives. 

 We supply a few snails in crystallising glasses with uninjured 

 leaves of Symphytum officinale or Boraya officinalis. At the same 

 time they are supplied with leaves of these plants which have 

 been deprived by means of a sharp knife of the rough pointed 

 hairs covering their surface. The latter leaves are readily eaten 

 by the snails ; they hardly attack the uninjured leaves at all. 



Leaves of Arum maculatum are not eaten by even very hungry 

 snails. The leaves are mechanically protected by raphides, which 

 at once pierce the mouth organs of the snails when they gnaw the 

 leaves, and so produce a highly unpleasant sensation. If we chew 

 small fragments of Arum leaf, we perceive an intense burning 

 taste, which is due to the raphides. Expressed unfiltered Arum 

 juice produces the same sensation when placed on the tongue, 

 while the sap freed from the raphides by filtration has only a 

 sweetish taste. 



See Stahl, Pfianzen und Schnecken, Jena, 1888. 



