EXCRETORY POWER OF THE ROOTS. 81 



V. Another function of tlie roots of plants^ in regard to which physiol- 

 ogists are divided in opinion at the present day, is what is called their 

 excretory power. 



1°. When barley or other grain is caused to germinate in pure chalk, 

 acetate of lime* is uniformly found to be mixed with it after the germi- 

 nation is somewhat advanced (Becquerel and Mateucci, Ann. deCliem. 

 et de Phys., 1 v., p. 310.) In this case the acetic acid must have been given 

 off (excreted) by the young roots during the germination of the seed. 



This fact raay be considered as the foundation of the excretory theory 

 as it is called. This theory, supported by the high authority of Decan- 

 doUe, and illustrated by the apparently convincing experiments of Ma- 

 caire, {Ann. de Chim. elde Phys., lii., p. 225,) has more recently been met 

 by counter-experiments of Braconnot, (Ixxli. p. 27,) and is now in a great 

 measure rejected by many eminent vegetable physiologists. It may in- 

 deed be considered as quite certain that the application of this theory by 

 DecandoUe and others to the explanation of the benefits arising from a 

 rotation of crops, is not confirmed, or j)roved to be correct, by any exper- 

 iments on the subject tliat have hitherto been published. f 



According to DecandoUe, plants, like animals, have the power of se- 

 lecting from their food, as it passes through their vascular system, such 

 portions as are likely to nourish them, and of rejecting, by their roots, 



* Acetate of lime is a combination of acetic acid or vinegar with lime derived from the chalk. 



1 The discordant results of Macaire and Braconnot were as follow : 



1°. Macaire observed that when pla.nls o{ Cfio7idrilla Muralis were grown in rain water 

 they imparted to it something of the smell and taste of opium. Braconnot confirmed this, 

 but attributed it to wounds in the roots which allowed tiie proper juice of the plant to escape. 

 He says it is almost impossible to free the younij roots from the soil in which tliey have grown, 

 without injuring them and causing the sap to exude. 



2°. Euphorbia Pejdiis (Petty Spurge) imparled to the water in which it grew a gummi- 

 resinous substance of a very acrid tuste. In the hands of Braconnot it yielded to tlie water 

 scarcely any organic matter, and that only slightly bitterish. 



3°. Braconnot washed the soil in which pianlsof Eup/ioibia Br eoni and Asdepias Incnr- 

 nata were growing in pots, and obtained ar solution containing earthy and alkaline salts with 

 only a trace of organic matter. 



He also washed the soil in which the Poppy (Papaver Somniferum) had been grown ten 

 years successively. The solution, besides inorganic earthy and alkaline .salts, gave a consid- 

 erable quantity of acetic acid (in the form of acetate of lime) and a trace of brown organic 

 matter. He infers that these several plants do not excrete any organic matter in sufficient 

 quantity to be injurious to themselves. 



4°. Macaire obseived that when separate portions of the roots of the same plant of Mercu- 

 rialis Annua were immersed in separate vessels, the one containing pure water and the 

 other a solution of acetate of lead,— the solution of lead was absorbed by the plant,— was to 

 be traced in every part of it, and afterwards was partially transmitted to the pure water. Bra- 

 connot observed the same results, but he found the entrance of the lead into the second vessel 

 to be owing to theascerUof the fluid up the outer surface of tiie one root and down the exterior 

 of the other, and that, by preventing tlie possibility of this passage, no lead could be detected 

 among the pure water. 



Tlie conclusions of Macaire, tlierefore, in favour of the rotation theory of DecandoUe 

 must be considered as at present inadmissible, and we shall hereafter see reason to coin- 

 cide, at least to a certain extent, in the conclusion of Braconnot, "tliat if these excretions 

 (of organic matter) really take place in the natural state of the plant, they are as yet so ob- 

 scure and so little known as to justify the presumption that some other explanation must 

 bo given of the general system of rotation." Various illustrations have been given by differ- 

 ent observers of this supposed excreting power of the roots. Among the most recent are 

 those oi Nietner, wlio ascribes the luxuriant rye crops obtained without manure after three 

 venrs of clover, to the excretions of this plant in the soil, which, like those of the pea and 

 Dean to the wheat, he supposes 'o be nourishing food to the rye. He also states that the 

 beet or the turnip after tobacco has an unpleasant taste, and is scarcely eatable, which he 

 attributes Lo the excretions of the tobacco plant. Meyen ascribes the effect of the clover to 

 (he green manure supphed by its roots and stubble and tliat of the tobacco to the undecom- 

 posed organic subsUnces contained in the sap and substance of the roots and stems of this 

 plant, of which so large a quantity is left behind in the Held.— [Meyen' s Jahresberickt, 18-39, 

 p. 5.]— These objections of Meyen are not without their weight, but we shall hereafter see 

 that they embody only half the truth. 



