with the words: "And now, to summarize in a few words the results of this whole discussion, I think the balance of the evidence points to the conclusion that the answer to the question, What are the sources of the nitrogen of vegetation in general, and of agricultural productions in particular? is more likely to be found in the relations of the atmosphere and of the plant to the soil than in those of the atmosphere to the plant itself."

On the whole, the evidence is decidedly against the theory so stoutly maintained by some, particularly Ville, that plants obtain considerable, some a large share, of their nitrogen by their leaves from the air.

Best Forms of Nitrogen for Plant-Food.

The important question as to the form of nitrogen most suitable for the nutrition of plants has been studied by Lehmann, who has lately experimented with buckwheat, maize, and tobacco, supplying nitrogen in some cases in the form of nitrates, and in others in the form of ammonia salts. He concludes that some plants require ammonia in their first period of vegetation, and nitric acid in the second, but that ammonia may, by oxidation in the soil, produce the nitric acid needed.

Hässelbarth has studied the assimilation of nitrogen by barley. The whole of the nitrogen was absorbed, and the total yield of dry substance the same, when the nitrogen was supplied as calcium nitrate or as ammonium nitrate in soil free from marl. In marled soil the nitrogen was wholly absorbed when supplied as ammonium sulphate or chloride. When the latter salts were applied to unmarled soils, only one half of the total nitrogen was absorbed. The worst re

sults were obtained by using acid ammonium phosphate: in marled soil only three fifths and in unmarled soil only one seventh of the nitrogen was absorbed, the yield being correspondingly small. It appears from these experiments as though barley could absorb the nitrogen only when present as a nitrate, or under conditions that permit of ready transformation to a nitrate (Chem. Centralblatt, 1876, 821).

It is worthy of note here that in the experiments of Lawes and Gilbert on grass land, nitrogen in the form of nitrate of soda, which finds its way into the deeper layers of the soil

more readily than in the form of ammonia salts, favored the growth of deep-rooted plants. Hence the herbage on plots manured with nitrate of soda stood the drought much better than that manured with ammonia salts.

Source of the Carbon of Plants.

The question as to the source of the carbonic acid from which the carbon of plants is obtained, whether it is derived exclusively from the atmosphere or partially from the soil, has been long discussed. Liebig was of the opinion that a portion was obtained from the soil through the roots, while Boussingault believed that the larger part, if not the whole, comes from the air through the leaves. Experimental data for the decision of the question have, however, been lacking. Boehm, from experiments with the scarlet-runner bean, concludes that young plants do not take up from the soil either organic compounds or carbonic acid. He considers it not improbable that the carbon of the carbonic acid decomposed by plants unites directly with water to form starch (Ber. d. D. Chem. Ges., ix., 123).

We have lately accounts of some very ingenious and interesting experiments by Moll, from which the author derives, evidently with good ground, several conclusions, of which the resultant is that plants derive none of their carbon from the carbonic acid imbibed from the soil through the roots (Landw. Jahrb., 1877, vi., 327).

Dehérain and Vesque have made some very ingenious experiments on the absorption and emission of gases by the roots of plants, from which they conclude that (1) the presence of oxygen in the soil containing roots is necessary for the existence of the plant; (2) the root connected with the stem evolves a quantity of carbonic acid less than the oxygen absorbed; (3) carbonic acid appears not to be derived from the soil, and does not pass to the leaves in order to build up proximate principles by its decomposition into carbon and oxygen (Comptes Rendus, lxxxiv., 959–961).

On the whole, the latest experimenting very decidedly favors the belief that plants get all the carbonic acid from which their carbon comes through their leaves, and none through their roots.

Phosphoric Acid as the Food of Plants.

Dr. Petersen, of the experiment station at Regenwalde, in Germany, reports some experiments in water-culture, with the object of determining what proportion of phosphoric acid is essential to the best development of the oat plant. The maximum yield was obtained in solutions which, besides the other essential ingredients of plant-food, furnished 0.071 grams of phosphoric acid to each plant. Plants grown in these solutions yielded each, on the average, 197 seeds, and in the whole plant 10,497 grams dry substance, or 316-fold the weight of the seed. In solutions exactly similar, except that they furnished only half as much phosphoric acid, the plants averaged only ninety-four seeds and 3508 grams dry substance, or 114-fold the weight of the seed.

The results of these experiments agree essentially with those of a number of similar ones reported some time since by Wolff. Both investigators found that when the phosphoric acid did not exceed 0.33 per cent. of the whole weight of the dry substance of the plant, the latter suffered in the development of all its parts. On the other hand, excess of phosphoric acid did no harm, but seemed rather to favor a better development of seed. Wolff calls attention to the difference between phosphoric acid and nitrogen in this respect, the latter, as is well known, having a tendency, when applied in excess, to injure the development of the seeds of cereal grains and cause an excessive growth of stalk, and often lodging of the grain. In this view, it is clear why the excessive use of phosphates which obtains in some farming districts has not proved injurious.

Lime as Plant-Food.

Boehm has studied the plant-nourishing value of calcium salts by experiments on scarlet-runner beans. He concludes that mineral matter is absolutely required for the young plant in order that it may avail itself of the excess of nourishment stored in the seeds, the mineral matter in the seeds being insufficient for this purpose; that without lime salts the growing plants soon wither away and die; and that, finally, lime salts do not participate directly or indirectly in the formation of starch. He also shows that plants may ab

sorb water and lime salts through their leaves (Chem. Cen tralblatt, 1876, pp. 250, 275, and 808).

ROOT-DEVELOPMENT OF PLANTS.

The study of the root-development of some of our more important agricultural plants is receiving increased attention of late. Very interesting observations have been made by Nobbe, Haberlandt, and Thiel. Fraas has published a little work on the subject; Müller has given a résumé of the main points of the present status of our knowledge of the subject in the Landwirthschaftliche Jahrbücher; and, finally, Von Nathusius and Thiel have issued a series of six charts, containing no less than fifty-three very fine photographs of roots of various plants as they actually grow in the soil. These include views of the roots of corn, barley, pease, Jerusalem artichoke, potato, and sugar-beet, from which the soil had been removed so as to allow of their being photographed as they grew. They show that while the fine roots penetrate much deeper into the soil than many suppose, yet by far the larger bulk are within a few inches of the surface, and that there most of the feeding of the plants through the roots is done.

Heinrich reports some interesting experiments on the development of roots of barley, oat, and pea plants. The plants were grown in boxes four meters (about thirteen feet) deep, filled with fine garden earth. The oat roots penetrated 2.27 meters; those of barley, 1.9; and of pease, only 0.52 meters. The soil was carefully washed away from the roots, and the latter, as well as the tops, weighed. The weight of the roots of oats was about two thirds that of the tops, without seed; those of barley weighed about one third, and of pease one fifth, as much as the tops.

Frémy and Dehérain find that sugar-beets grown in saline solutions, instead of sand moistened with the same, lived; but instead of producing one large central sugar-forming root, they simply formed a mass of nearly equal rootlets.

MANURES.

Phosphatic Fertilizers.

Dr. Voelcker has continued his reports on these materials, giving analyses of over fifty samples of phosphatic guanos.

Exposed to weather in various climates, bird-dung undergoes change, and guanos of various qualities are produced. The ultimate effect of rain is to destroy the organic matter; the guano then remaining is said to be phosphatic, and contains very little nitrogen. These phosphatic guanos are of great value as materials for high-grade superphosphates. In many cases the bases present are insufficient to form tribasic salts with phosphoric acid, and the phosphoric acid is hence in more readily available forms. The principal supplies are from South America, South Africa, a number of islands in the Caribbean Sea, several uninhabited islands in the South Pacific, and Raza and other islands in the Gulf of California. The richest samples analyzed were those from Raza Island, in the Gulf of California; and Shaw's, Enderbury, Starbuck, and other coral islands in the Pacific. These yielded from 32 to 40 per cent. of phosphoric acid. The latter are, however, apt to be contaminated with coral rock. The Baker, Howland, and Jarvis islands are nearly exhausted. Mejillones guano, a deposit near the coast of Bolivia, is estimated at several million tons. Considering the fact that phosphoric acid is the ingredient most apt to be deficient in our soils and most largely needed in commercial fertilizers, the reports of Dr. Voelcker, which show that immense supplies of this material are accessible in various parts of the world, are very cheering (Journal Royal Ag. Soc., xii., 440–459).

Nitrogenous Fertilizers.

Dr. Voelcker reports also a number of unweathered or partially weathered guanos from new deposits on the South American coasts, which indicate that the supply of these is far from being exhausted. A number of analyses are given in Vol. XIII., pp. 194 sq., of the journal just referred to.

Analyses of eleven samples of Peruvian guano are given in the "Report of the Connecticut Agricultural Experiment Station for 1876." They averaged better than was claimed by the sellers: "Of over two hundred samples of fertilizers analyzed at the station, leaving out a few articles of at present mere local importance, like crude fish-scrap, no others have been found which, as a class, taking into account both quality and price, furnish the valuable ingredients of plantfood so cheaply as Peruvian guano." A great step in ad