654 



THE SOUTHERN PLANTER. 



[November 



osmotic diffusion could not possibly explain. 

 The alumina and the silica which were not 

 required by the plant, would be excreted by 

 the roots on the assimilation of the ammo- 

 nia. We thought that it was far more con- 

 sistent with similar phenomena that the de- 

 composition of the ammoniacal compounds 

 should be effected by the affinity existing be- 

 tween them and the substances of the cell- 

 walls or cell-contents. Like carbonic acid 

 by the leaves, the ammonia would be sepa- 

 rated from the other non-nutritive substances 

 forming compounds with it, and drawn into 

 the cells by chemical affinity. Silica, phos- 

 phoric acid, and some other substances, we 

 still consider, are in this manner dissolved 

 by the roots of plants and find their way 

 into their structure. 



In the new views which we are now 

 forced to take of the absorbing powers of 

 soils, the question of the action of the roots 

 on ammonia and potash is greatly simplified. 

 These, being merely adhering to the surface 

 of the substances constituting the body of 

 the soil, are by no means out of reach of the 

 direct action of the roots of plants. The 

 roots come in contact with them in forcing 

 their way through the soil, and take them 

 up by special absorption, in the same way as 

 the leaves of plants do carbonic acid from 

 the atmosphere. The principle of the dif- 

 fusion of the gases in the atmosphere is not 

 the most active means of bringing carbonic 

 acid in contact with the leaves, although 

 we might suppose it capable of doing so to 

 a slight extent. The circulation of the at- 

 mosphere, in being constantly disturbed by 

 winds, accomplishes this — the rapid bring- 

 ing of the nutritive substances in contact 

 with the leaves. So, in the soil, the roots 

 are constantly extending in search of food ; 

 and the rains, as they descend, greatly as- 

 sist in diffusing it equally over the absorb- 

 ing surfaces. 



That, however, the food of plants exists 

 in the soil in forms so slightly soluble in 

 water as to be totally incapable of being 

 dissolved by it and supplying the wants of 

 plants, has been forcibly put by Liebig : — 



Let us assume that on a 2J-acre field 

 12,000,000 lb. of rain-water fall in a year, 

 and that a third of this quantity dissolves 

 from the soil the same ingredients, in the 

 same proportions, as in the drainage-water 

 analysed by Way. Let us further suppose 

 that these 4,000,000 lb. of water are, dur- 

 ing the months of June, July, August and 



September, completely absorbed by the roots 

 of a crop of potatoes, and again evaporated 

 from the leaves; then it follows that, on four 

 fields of 2 J acres each, the whole crop of 

 potatoes would not receive a single pound of 

 potash ; on two others of the same size they 

 would obtain rather more than a pound; 

 and on a seventh 2£-acre field, 2 lb. Now, 

 from an average crop of potatoes on a 2}- 

 acre field, there are obtained 408 lb. of 

 ashes, in which are contained 200 lb. of 

 potash. 



Brustlein still refers to the experiments of 

 Hales, as well as of Lawes, in support of the 

 idea that plants may take up all the food 

 they require in a soluble state. It ought 

 to be borne in mind, that plants do not take 

 up earthy substances in proportion to the 

 amount of water that they respire. Indeed 

 it is often quite the reverse — a circumstance 

 which is the strongest argument in favour 

 of special absorption, as well as the dissolv- 

 ing action of the roots. Plants perspire less 

 when the dew-point of the air is high, and 

 when evaporation is consequently weak. It 

 is then, however, that growth is most active, 

 and that the earthy substances must be taken 

 up in the greatest quantity. For the same 

 reason, mangold-wurzel, which resists drought 

 better than turnips, can take up a much 

 larger quantity of food in proportion to the 

 water that it evaporates. So also in water- 

 plants, in which the circulation of the fluids 

 is so much slower than in land-plants; the 

 food, being in solution, is especially absorb- 

 ed by leaves as well as roots. There is no 

 greater difficulty in accounting for the spe- 

 cial absorption of the nutrient matters by 

 the roots than that of the carbonic acid by 

 the leaves from the atmosphere. Indeed, 

 the chief nutrient matters are only retained 

 by the soil by mechanical attraction or ad- 

 hesion, while the roots have a chemical af- 

 finity for them. The weaker force merely 

 yields to the stronger on absorption by the 

 roots taking place. The vital force is a di- 

 recting power, no doubt ; but all changes, 

 in both living and dead organisms, are ef- 

 fected by chemical affinity. 



It is in the discussions of problems which 

 lie just beyond those connected with the ab- 

 sorbent powers of soils, that the practical 

 agriculturist is more particularly concerned. 

 In order that plants should exercise those 

 functions which we attribute to them, it is 

 essential that the soil should contain no mat- 

 ters interfering with the healthy action of 



