84 CAN THE ROOTS MODIFY THE FOOD OF PLANTS? 



ing it in the vascular system of the plant, and again returning to the 

 soil for a fresh supply. In either case the roots must have allowed it 

 egress as well as ingress. But the fact, that the proportion of silica in 

 the plant goes on increasing as it continues to grow, is in favour of the 

 latter view — and renders it very probable that the same quantity of al- 

 kali returns again and again into the circulation, bringing with it sup- 

 plies of silica and probably of other substances which the plant requires 

 from the soil. And while this view appears to be the more probable^it 

 also presents an interesting illustration of what may probably be the 

 kind of function discharged by the potash and other inorganic substances 

 found in the substance of plants — a question we shall hereafter have oc- 

 casion to consider at some length. 



The above considerations, therefore, to which I might add others of a 

 similar kind, satisfy me that the roots of plants do possess the power of 

 excreting various substances which are held in solution by the sap on its 

 return from the stem — and which having performed their functions in 

 the interior of the plant are no longer fitted, in their existing condition, 

 to minister to its sustenance or growth. Nor is it likely that this excre- 

 tory power is restricted solely to the emission of inorganic substances. 

 Other soluble matters of organic origin are, no doubt, permitted to es- 

 cape into the soil — though whether of such a kind as must necessarily 

 be injurious to the plant from which they have been extruded, or to such 

 a degree as alone to render a rotation of crops necessary, neither reason- 

 ing nor experiment has hitherto satisfactorily shown. 



VI. The roots have the power of absorbing, and in some measure of 

 selecting, food from the soil — can they also modify or alter it as it passes 

 through them? A colourless sap is observed to ascend through the 

 roots. From the very extremity up to the foot of the stem a cross sec- 

 tion exhibits little trace of colouring matter, even when the soil contains 

 animal and vegetable substances wjiich are soluble, and which give dark 

 coloured solutions, [such as the liquid manure of the fold-yard.] Does 

 such matter never enter the root ? If it does, it must be speedily changed 

 or transformed into new compounds. 



We have as yet too few experiments upon this subject to enable us to 

 decide with any degree of certainty in regard to this function of the root. 



It is probable, however, that as the sap passes through the plant, it is 

 constantly, though gradually, undergoing a series of changes, from the 

 time when it first enters the root till it again reaches it on its return from 

 the leaf. 



Can we conceive the existence of any powers in the root, or in the 

 whole plant, of a still more refined kind ? The germinating seed gives 

 off acetic acid into the soil, — does this acetic acid dissolve lime from the 

 soil and return with it again, as some siippose (Liebig), into the circula- 

 tion of the plant?* Is acetic acid produced and excreted by tlie seed 

 for this very refined purpose? We have concluded that in tlie wheat 

 plant the potash and soda probably go and come several times during its 

 growth, and the ripening of its seed. Is this a contrivance of nature to 



• Braconnot found acetate of lime in very small quantitios to be singularly hurtful to vege- 

 tation, and acetate of magnesia a little less so. He only mentions, however, some experi- 

 ments upon mercurialis annua, [Ann. de Chim. et de Pliys. Ixxii. p. 3G,] and as Saussure 

 found that some plants actually refused to take it up at all, these acetates may not be equally 

 injurious to all plants. 



