Comparative Physiology. 559 



plant inserted ; evidently pointing out that the fibres had ex- 

 erted some influence in dissolving as w^ell as absorbing the 

 food. As I mentioned before, it is probable that the excretion 

 of some nitrogenous substance from the roots (nitrogenous 

 substances, as diastase, &c., being powerful solvents) greatly 

 assists in such action ; though the contact of the roots, further- 

 ing the removal of carbonic acid, &c., as formed, may be part of 

 the cause also. The spongioles may also act, as living vessels 

 are generally supposed to do, on their food by attraction ; and 

 their vital pov^ers may produce some action in vs^hich electricity 

 is concerned, though not to such extent as the assimilating 

 organs. There is no proof, however, of streams of electricity 

 issuing to decompose the food, as supposed by some. It is evi- 

 dent also, that the food exerts some influence on the direction of 

 the roots, perhaps something of the nature of stimulus. Mr. 

 Knight, in his experiments, found that, when he buried the ma- 

 nure deep in the ground, the roots of the plants sown on the 

 surface descended at once in a spindle form, without any ten- 

 dency to ramify ; but when he sjjread the manure near the sur- 

 face, the roots of the same kind of plant ramified in all directions 

 in quest of the food. He found, also, that, when he made 

 trenches of loose ground in the midst of solid ground, the roots 

 preferred ramifying in the loose soil to penetrating the solid, 

 showing their tendency to extend in the direction of least resist- 

 ance. More food would also be found in such situations. In 

 quaggy miry land, the roots of the plants that thrive thereon 

 are found to prefer running along the surface to obeying the 

 natural impulse of gravity, though no resistance is presented to 

 their penetrating downwards, preferring the situation where 

 food of the best description is to be got. Mr. Knight considered 

 that gravity was the cause of roots descending, which impulse, 

 he said, tliey would always be found to obey, unless prevented 

 by resistance or the presence of food ; such instances as the 

 al)ove, of roots travelling long distances perpendicularly and 

 horizontally in quest of earth, he explained on these principles. 

 In the experiments above narrated on manure buried deep, 

 and merely pointed into the surface, Mr. Knight found the crop 

 on the portion manured on the surface to exceed that where it 

 was buried deep in an immense degree ; showing the benefit of 

 feeding seedling crops as soon as possible after the evolution of 

 roots. Much of the success of seedling crops, especially such 

 as turnips, depends on starting them with vigoui\ Of the bad ef- 

 fects of tannin on the roots I have had experience in cauliflower 

 and other plants sown above hot tanner's bark, when deficient 

 of fermenting stable manure. The plants vegetated strongly, 

 and had a good appearance, but, after pushing the third or fourth 

 leaf, set up altogether in growth, and got hard and woody in 



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