344 PHYSIOLOGY OF NUTRITION. 



easily distinguish, the cork, the cortex, and the bast and wood of 

 the vascular bundles. In the somewhat older parts of the root 

 the wood forms a closed cylinder traversed by medullary rays. 

 If the sections are treated with dilute potash solution, the contents 

 of the thin- walled elements of the cortex and phloem take on an 

 intensely red colour, which is very stable (Chrysophanic acid 

 reaction.) 4 



1 See Errera, Botan. Centralblatt, Bd. 32, p. 71. 



2 Eosoll, Sitzungsber. d. Akadem. d. Wiss. zu Wien, Abthl. 1, Bd. 89. 



3 An exact account of the structure of the Berberis stem, and especially of 

 its cortex, will be found in a Konigsberg Dissertation by Boning, of the year 

 1885, Ueber die Anatomic des Stammes der Berberitze. 



4 See Borskow, Botan. Zeitung, 1874, and 0. Herrmann, Lcipziaer Disserta- 

 tion, 1876. 



137. The Bye Products of Plant Metabolism as Means of 

 Protection to Plants. 



Many creatures, particularly mammals, insects, and snails, 

 would be exceedingly prejudicial to the life of plants, if these 

 were not more or less protected. The means of protection are 

 partly mechanical, partly chemical, and we often find, though cer- 

 tainly not always, that plants which are protected mechanically, 

 do not produce any so-called chemical means of protection, and 

 vice versa. 



Many bye products of plant metabolism, as has already been 

 pointed out several times, have especially the object, among other 

 things, of providing the plant with a chemical protection against 

 animals, and we will here, with Stahl's researches 1 as a basis, 

 undertake a few experiments by means of which this fact can 

 easily be proved. Our object is to show that tannins, vegetable 

 acids, ethereal oils, etc., afford the plant protection from snails. 



We collect a number of snails, e.g. Helix pomatia and H. 

 hortensis. For two to three days the snails are not supplied with 

 food, so that when the experiments proper begin they may be 

 very hungry. They are now placed in good-sized crystallising 

 glasses, two or three in each if Helix pomatia is used, larger 

 numbers if smaller kinds of snails are used, and the glasses are 

 then covered with glass plates, which are weighted, so as to 

 prevent the snails from creeping out. The snails are now sup- 

 plied with various kinds of foods. We will experiment with the 



