Abstract
In addition to water and gases, developing plants need minerals from the soil, even though these substances occur in very small amounts in plants. From ten test solutions, each containing the same amount of a different mineral constituent, roots took up varying amounts of minerals, but absorbed none of them in greater proportion than the water of solution. Variations in assimilation also occurred for solutions containing several mineral solutes. Roots absorbed toxic as well as beneficial substances. The minerals that were absorbed varied with the plant species. The dry weight of the extract that the most fertile soil can produce is much less than the dry weight of a plant that develops there, so the plant clearly can be taking only a very small percentage of its dry matter from that source, even though such substances are essential for growth.
Copper sulfate is toxic, killing the roots and thus destroying their selective power of absorption. Sluggish growth, and the cutting off of roots, also promote the unrestricted absorption of solutes.
Variations in assimilation are attributable largely to differences in viscosity. The less viscous a substance, the more easily it penetrates the root. Gum, which yields a very viscous solution, is assimilated to a much lesser extent than is sugar.
Most of a plant's substance is derived from carbon dioxide and water. The roots of green plants absorb only a very small amount of salts and other constituents from the extractive and saline solutions of the humus, yet these constituents have a very pronounced effect on plant growth.
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Notes
- 1.
For a month I nourished thirty Polygonum persicaria and peppermint plants with distilled water to which I had added a certain weight of finely divided silica, which remained partly suspended with the aid of a small amount of sugar dissolved in the liquid. After the experiment, I did not find, either by incinerating the plants or by closely examining the residue of the absorbed liquor, that this earth had penetrated the plant appreciably. Bonnet caused some plants to absorb ink, but only an imponderable amount of the undissolved coloring matter was absorbed. It would have passed much more profusely through the most perfect filters we can make.
- 2.
I think it is unnecessary to list here the experiments of van Helmont, Tillet, Bonnet, and Duhamel, who were the main supporters of this theory. The flaws in their methods were sufficiently demonstrated by Bergman, Kirwan, and Hassenfratz. They noted especially that the vessels in which Tillet and van Helmont grew plants were porous and buried in plant mold, which could influence the results these authors obtained. Duhamel's oak was moistened with water naturally rich in extractive material. Bonnet's plants had for support plant substances more or less soluble in water.
- 3.
Bibliothèque Britannique. (Mémoires sur la culture du Blé. Agriculture, Vol. 5, p. 499).
- 4.
I did not dissolve previously prepared and dried extract in water because, if this is done, the solution still holds in suspension the part of the extract that was precipitated during evaporation. Instead I made a cold infusion of humus in water. The evaporation of a part of this infusion showed me that the infusion in which I grew the plants contained 25 parts of extract. The results obtained with salts are more accurate than those with extract, sugar, and gum because these plant compounds are always a little corrupted by contact with the roots.
- 5.
The saline solutions were analyzed more accurately by reagents than by any other method. I ascertained that the 100 parts of potassium chloride that I used formed, by silver nitrate, a precipitate equal to
187-1/2 parts
100 parts of sodium chloride, by the same reagent
232-1/2 parts
100 parts of ammonium chloride, by the same reagent
270 parts
100 parts of calcium nitrate, by potassium oxalate [“oxalate of potash”]
78 parts
100 parts of calcium acetate, by the same reagent
81-1/4 parts
100 parts of powdered sodium sulfate, by barium acetate [“acetate of Baryta”]
167-1/3 parts
100 parts of copper sulfate, by the same reagent
94-1/2 parts
The solutions of gum, sugar, and humus extract were evaporated to dryness and the residue was weighed.
- 6.
I precipitated the residue of root suction by barium acetate [“acetate of Baryta”]; the decanted liquor was precipitated by silver nitrate
- 7.
The analysis was done by the same procedure. It is evident that I made plants take in salts that mutually decompose. But this decomposition was not carried out here because the solutions were too dilute. Berthollet showed that the exchange of substances took place only when caused by the force of crystallization, or, what amounts to the same thing, by the insolubility of the new combination
- 8.
The residue of the root suction was divided into two equal parts; one was precipitated by potassium oxalate [“oxalate of potash”] and the other by silver nitrate
- 9.
The residue of root suction was divided into two equal parts; one was precipitated by potassium oxalate, and the other by barium acetate. After the separation of the barium sulfate [“sulfate of Baryta”], the decanted liquor was precipitated by silver nitrate
- 10.
I evaporated the residue to the consistency of syrup and then threw on it alcohol, which dissolved the sugar and precipitated the gum. A comparative experiment showed me that the precipitated gum obstinately retained about a quarter of its weight of sugar. The results that I have given were corrected according to this principle.
- 11.
For these effects to be detectable, the solutions must be more concentrated than the ones that I let plants absorb. It seems likely to me that a dilute aqueous saline solution is not always a homogeneous combination, but a mixture of saturated and unsaturated water. It is well known that a dilute aqueous solution of sodium chloride that is left completely at rest becomes denser and saltier at the bottom of the vessel than at its surface. (See the memoir of Leblanc on Cristallotechnie. Journal de Physique, year 11 [of the French Republican calendar; i.e., 1802].
- 12.
In gourd plants, this malady begins with viscous beads that appear especially on the upper surface of the leaf, near the petiole. These beads dry and form prominent, powdery white blotches that spread and multiply successively, out to the perimeter of the leaf. I detached such an incrustation. It was only partially soluble in water and in alcohol. These solutions, evaporated to dryness, yielded a deliquescent salt that had all the properties of calcium chloride combined with a negligible amount of magnesia. The salt was precipitated in abundance by silver nitrate, potassium oxalate, and alkaline carbonates, but not at all by barium hydroxide fully dissolved in water [“water of baryta”], and it was almost unalterable by fire. The saline and earthy part of the incrustation made up about a third of its weight. It was enveloped by a white plant substance insoluble in water and alcohol, and abundant enough that the incrustation itself did not noticeably draw humidity. This malady especially attacks old plants growing in soil that is very rich in animal manure, and in hot-beds where the leaves are not washed by rainwater.
- 13.
Traité de Minéralogie, by Haüy, Vol. 1, p. 7.
- 14.
I could rely on the experiment of van Helmont, who, after having grown a willow for 5 years in 200 pounds of humus, found that this plant had gained, in the green state, 164 pounds and that the humus, which was dried in an oven before and after the experiment, had lost only 2 ounces. But Kirwan made some very valid reflections on these results: He observed (1) that the humus was placed in an unglazed, porous earthen vessel, which was itself buried in plant mold, and that this last conveyed extractive juices to the plant; (2) that the oven drying might not have been the same before and after the experiment; (3) that the willow left, in the soil, root fibrils whose weight could not be estimated; (4) that the rain water that was used for watering the plant must, through its impurity, have contributed to the plant's nutrition. (Mémoire sur les Engrais, by Kirwan.)
- 15.
Hales says that a sunflower in full growth loses only a quarter of its weight by drying. This result is certainly incorrect, as are the inferences that this author deduced from it. [De Saussure misinterprets Hales, who wrote, on p. 8 of Vegetable Staticks (1727), that the sunflower, upon drying, lost ¾ of its weight as water.]
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Hill, J. (2013). Of the Absorption of Solutions by Plant Roots. In: Chemical Research on Plant Growth. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-4136-6_8
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