FOOD OF PLANTS. 



561 



of the peptic glands of animals, and is only efficient when associated 

 with an acid ; so that the solution of nitrogenous matters "by the leaves, 

 pitchers, and other organs of certain plants is now shown to be a true 

 digestive process, resembling in every particular the corresponding pro- 

 cess in the intestinal canal of animals. 



Ammonia salts have also been found to be rapidly absorbed, and to 

 excite the sensitiveness of the plants to an extraordinary extent : still the 

 actual benefit to the plant seemed doubtful ; for to ordinary observation 

 these so-called carnivorous plants appear to thrive quite as 'well without 



Fig. 589. 



Fig. 590. 



Fig. 589. Cells of petal of Helleborus prior to application of meat. 

 Fig. 590. Cells of petals of Helleborus after the application of meat. 



as with nitrogenous diet. To settle this point, Mr. Francis Darwin 

 experimented on a large number of plants of Drosera grown under like 

 conditions, half of them fed, the other not receiving artificial supplies. 

 The general result, given in the following Table, shows the great benefit 

 derived by the fed plants, especially in the formation of seeds : 



Results of Experiments on Drosera. 



Unfed. Fed. 



Total weight of plants, excluding flower-stems 100 122 



Total number of flower-stems 100 16-5 



Sum of heights of flower-stems 100 1GO 



Total weight of flower-stems 100 232 



Total number of capsules 100 194 



Average number of seeds per capsule 100 123 



Total weight of seeds 100 242 



Total number of seeds 100 380 



Sources of Mineral Food. For their mineral food, plants are 

 of course chiefly dependent on the soil in which they grow. The 

 gradual decomposition of rocks furnishes the earthy and alkaline 

 constituents, which must vary on different formations or according 

 to diluvial actions. Marine plants naturally accumulate many of 



