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|Sheffield Scientific School
New Haven, Conn., Feb. 25, 1873.
Editor Popular Science Monthly.
Dear Sir: In his interesting article on "Spontaneous Movements in Plants," printed in your January number, Dr. A. W. Bennett remarks, page 284, that "the selective power of plants, in absorbing from the soil a larger portion of those ingredients which are required for the formation or healthy life of their tissues, is an absolutely unexplained phenomenon." Dr. Bennett says further, "A striking instance of the liability to consider a mere statement of an obscure law in other terms as an explanation of that law, occurs in an admirable treatise on the growth of plants—Johnson's 'How Crops Grow.'" Then follows the subjoined quotation from my book (the italics are Dr. Bennett's): "The cereals are able to dispose of silica by giving it a place in the cuticular cells: the leguminous crops, on the other hand, cannot remove it from their juices; the latter remain saturated, and thus further diffusion of silica from without becomes impossible, except as room is made by new growth. It is in this way that we have a rational and adequate explanation of the selective power of the plant." Dr. Bennett adds: "The 'rational and adequate explanation' seems to me, on the contrary, to be merely a restatement of the selective power of the tissues in other terms. Because the tissues want the silica, is no explanation of how they get it."
Very possibly, Dr. Bennett holds me at a disadvantage as the matter thus stands, but I am, in fact, very seriously misrepresented in the last sentence of his quotation from "How Crops Grow." On page 363 of the American edition, the reader may see that the period which concludes Dr. Bennett's quotation should be a comma, and that the sentence, as I wrote it, first comes to a conclusion after an important qualifying clause, and reads, entire, as follows:" It is in this way that we have a rational and adequate explanation of the selective power of the plant as manifested in its deportment toward the medium that invests its roots."
It appears that Dr. Bennett has inadvertently confused two quite distinct things. He asserts, at first, that "the absolutely unexplained phenomenon" is "the selective power of plants in absorbing from the soil those ingredients which are required for their tissues." But, afterward, he declares my "explanation" to be "merely a restatement of the selective power of the tissues." Obviously, the selective power of the plant, as manifested toward the medium that invests its roots, is one thing, and the selective power of the tissues toward the substances dissolved in the cell-juices is, or in many cases may be, another. The former is what I offered a rational and adequate explanation of. The latter I have not ventured to attempt to explain. The former is explained by being coordinated with the well-ascertained facts of "diffusion" and "osmose," and referred to established, if not fully-developed, physical laws. The latter belongs to the yet very obscure phenomena of chemism, which are only known to us imperfectly in some of their results, and whose inner nature the recent amazing progress of organic chemistry has hardly begun to enable us to speculate upon with any satisfactory degree of probability.
Dr. Bennett writes of silica as one "of