THE INNER LIFE OF PLANTS. 325 



discover that certain plants have likewise developed tastes and 

 predilections for special kinds of food. For example, it is interesting 

 to find that some plants will not flourish unless zinc is included in the 

 list of substances constituting their dietary. This metal is ordinarily 

 unknown in the list of food-stuffs demanded by plants ; yet Viola 

 calaminaria and Thlapsi calaminaris present us with examples of plants 

 for which zinc is a necessity in so far as healthy growth is concerned. 

 Whilst a minute quantity of iron is necessary, as already noted, for 

 plant-growth at large, certain plants appear to demand much larger 

 quantities of this metal than are ordinarily supplied by the soil. 

 Maize is an example of those plants, for the healthy growth of which 

 iron appears to be an absolute necessity ; and buckwheat will not 

 grow unless the elements potassium and chlorine are supplied. The 

 list of special proclivities in the way of choice of unusual food- 

 ingredients by plants might be well-nigh indefinitely prolonged. 

 Enough has been said, however, to show that there operate in the 

 world of plant-life habits and conditions determining food-supply 

 strikingly analogous to those which cause the animal to prefer one food- 

 material, and to reject another. That this selective power in plants 

 depends on what may be familiarly named "constitutional peculiarities " 

 appears tolerably evident from the results of experiments upon the 

 absorptive power of different plants when tested by the offer of a 

 varied range of material. Certain plants (e.g. Mtrcurialis annua) 

 have been known to exhibit a striking preierence for nitre when 

 that substance was mingled with common salt ; whilst, on the other 

 hand, a species of Satureia absorbed salt, but rejected the nitre. 

 Arsenic, as a rule, is fatal to vegetable life ; yet some fungi have been 

 known to grow in solutions of this substance, exhibiting thus an 

 adaptation to circumstances as typical as that afforded by any living 

 form. This selective power, which forms such a marked feature in 

 the inner life of plants, possesses naturally an economic and practical 

 interest for the agriculturist. The " rotation of crops " practised by 

 the farmer, is the result of a knowledge of the fact that one species of 

 plants prefers what another species rejects ; and it is the absence of 

 the knowledge or the lack of attention to its teachings which has 

 made the once fertile fields of Sicily and Spain utterly unproductive 

 in the present epoch. 



Far exceeding in interest the foregoing details respecting the 

 development in plants of a predilection for special kinds of food, 

 are facts (which the patient industry of Mr. Darwin was mainly 

 incidental in bringing to light) respecting the extraordinary habits of 

 certain species of higher plants which feed upon organic matter, and 

 which appear to prefer such material when drawn and captured from 

 the world of animal life. No more typical instances of the develop- 

 ment of a special "habit" in plants could well be cited than the 



