The discussion about the "insect eating" plants, has brought this in as aside issue, though one of far more practical importance to horticulture than the main question.

I know that the position I am about to take on this question is not only in opposition to that held by yourself, and your correspondents, Prof. C. Y. Riley, and Mr. Thos. Forfar, but many others. But let us see how far such opinions are borne out by the following broad facts.

Within six miles of where I write, we have three large manufactories of fertilizers; one an immense ground bone manufactory at Newark, N. J.; another, the Lodi Poudrette manufactory situated on the Hackensack Meadows; the other, the manufactory for the Blood and Bone Fertilizer, located right in the midst of our market gardens at Communipaw, N. J., all of which are within ten miles of the city of New York. The effluvia from these manufactories is carried in the air for miles, and in the immediate vicinity is almost unbearable, yet if any one think that the meadow grass in the vicinity of the Poudrette factory, or the vegetables grown contiguous to the blood and bone establishments are benefited by the "absorption," any more than the same class of vegetation a dozen miles away, he will be quickly undeceived if he will take the trouble to examine. I have critically examined them on several occasions, and at all seasons, in company with some of our most experienced market gardeners here, and the result has ever been the same, no difference whatever was apparent either to grass, trees or garden vegetables in the closest proximity to the works.

Now if our examination was correct, and a verification of it can easily be made any Summer's day, it would seem that the effect of "putrid beef tea placed under a stand of plants," as suggested by your correspondent, Mr. Thos. Forfar, would be very slight indeed.

Prof. Riley says that "as a practical gardener Mr. Henderson will not deny that many plants with tender foliage may be nourished and in fact frequently are nourished, by the application of liquid manure to their leaves." To this I reply that my practice which has extended over a period of over thirty-five years, and which I believe has been as varied and extensive as that of most men in that time, has never yet shown me a single instance wherein I was certain that plants either absorb liquid manure, or even fertilizing gases by their leaves.

It is the easiest thing in the world to "mistake causes." The school of medicine, less than fifty years ago bled and blistered mankind for nearly all the ills that humanity is heir to; today the newest Hedged saw-bones thinks he knows better, and looks pityingly on the San-grados of half a century ago! So may not some of us, (who are certainly far from being as intelligent a class as the medical profession), have got off on a wrong tangent? Even when you, Mr. Editor, assume that the rank growth of plants grown in hotbeds, over those grown in other artificial heat, is due to the absorption of nitrogenous matter through the leaves, it is possible you may be mistaken in your assumption, for no* doubt you have seen in pine pits where tan bark was used for plunging, a luxuriance of growth quite equal to that of any hotbed formed of dung, and yet we know that the exhalations from tan bark are not likely to contain either ammonia or nitrogen. The famous Boston hotbed-grown lettuce, by which our New York markets were almost exclusively supplied ten years ago, now finds a formidable rival in the lettuce grown in greenhouses, hundreds of which, covering many acres are now in use for that purpose in nearly every Northern State. Budlong & Co., Providence, R. I.; Abraham Van Sicklen, Jamaica, L. I.; Mabbitt & Wills, of Riverton, N. J., and Muir Bros, of Newark, N. J., are all extensive greenhouse growers of lettuce for the New York market; and I am informed by a leading commission dealer, who sells for the two firms first named that their lettuce now far exceeds in quality the hotbed-grown Boston lettuce, though no manure in any shape is used except at the roots.

My belief is, that it is the uniformly proper condition of moisture that we get in a dung hotbed, that gives us the luxuriance of growth, for when we produce the same conditions in the atmosphere of the greenhouse, we know that we attain equally good results. That leaves absorb moisture from the atmosphere is so generally a received opinion that when I question its truth, I well know that I am trenching on dangerous ground, but let us examine. If we take a plant from a greenhouse that is kept continually charged with moisture and place it for a few hours in a dry room its leaves begin to wilt and droop. Now the believer in the moisture-absorbing theory will say that this is because it has not sufficient moisture in the dry air for its sustenance, but is it not just as easy to suppose that it is because the plant evaporates its moisture in a dry atmosphere, while it would not do so in a damp one ? Or in other words the moist air acts only as a negative benefit, preventing the evaporation of the moisture from the leaves which has been taken from the soil by the roots.

I believe that you can no more feed a plant through its leaves, than you can feed man, or any other animal, for any length of time through the pores of the skin, and that it is from the roots and root only, in plants that absorption can take place to any noticeable extent.

In experimenting on this subject a few years ago I suspended two large stems of Cactus triangularis and C. grandiflorus, in a moist stove, weighed them when put in, and at the end of six months weighed them again, and though they kept fresh and green, they had neither increased nor diminished in weight, though the air was pregnant with moisture and often ammonia too; for we use liquid manure largely in all our greenhouse operations, but we use it at the roots only, and if by chance it gets on the leaves we at once wash it oft' with the hose, believing it to be injurious rather than otherwise, in clogging the pores.

When a boy, I was for some time clerk in a liquor store in the city of Edinburgh. When whiskey was sent to us from the distilleries it had to be pumped out of the puncheons into smaller kegs for distribution. On every pumping day our old Highland porter invariably got drunk, but he always protested he was a victim of circumstances, a martyr to duty; the smell from the whiskey made him "light headed." It was imagined for some time that Donald might have been so constituted that the fumes affected him and no one else, but we had him watched, and found as we had before suspected, that he was taking his stimulant through his mouth to his stomach; so I rather suspect it will be found in all such cases as Professor Riley alludes to, that when plants are supposed to be stimulated by application of liquid manure to the leaves, that enough may have run from the leaves to the roots, the stomach (?) of the plant, to stimulate growth.

[We offered the hotbed observations not as direct proof, but as collateral evidence. As to the main point, can plants absorb and digest nitrogenous matter through their leaves ? We know they do so use carbonic acid in that way, and there is nothing therefore impossible in their using other elements in like manner and that they really do use nitrogen through their leaves we think is made a certainty to most persons who will carefully study Mr. Darwin's facts as given in uInsectivorous Plants."]