Boussingault, working at the same time, 1837-1855, but experimentally with the living plant, taught us more about these matters than any investigator of the time, though it is very probable that the stimulus of Liebig's speculations, good and bad, had its effect in impelling Boussingault to devote his splendid methods to problems of plant-nutrition. Boussingault's contributions to our knowledge of the composition of the dead plant cannot be over-estimated; but he did more than this, because he so clearly apprehended the necessity for asking his questions directly of the living plant, instead of deducing from chemical principles what might be supposed to occur in it; and although future researches showed that even so careful an investigator solved a problem of first importance--viz. the question of the fixation of free nitrogen--the wrong way, it will be found that so far as he did go his conclusions were sound, and well calculated to inspire the confidence with which the world received them. As we are here concerned more especially with the botany of agriculture, however, it is unnecessary to dwell longer on these matters, or on the similar and even more extensive experiments, of world-wide reputation, carried on for so many years, and still being carried on under the liberal auspices of Sir John Lawes, at Rothamsted. Moreover it may be necessary to return to some of these points later on. NOTES TO CHAPTER I. The reader will find a further general account of these matters in Sachs' _Lectures on the Physiology of Plants_, especially Lectures I. and XII., Engl. ed., Oxford, 1887. He may then proceed to Pfeffer's _Physiology of Plants_, Engl. ed., 1899, chapter I., and to the account of the history of the subject in Sachs' _History of Botany_, Oxford, 1890, especially pp. 359-375 and 445-524. References to more special literature will be found in Pfeffer. CHAPTER II. THE PLANT AND ITS FOOD. _The food of plants--"Vital force"--Other errors--Liebig and Boussingault--The botany of agriculture. The synthesis of carbohydrates--The physiology of plant-nutrition. The persistence of misconceptions._ The year 1860 may be regarded as a landmark of importance in the history of plant physiology, for it was in that year that Sachs discovered that the bringing together of water and carbon-dioxide, in the green chlorophyll-corpuscles of the plant exposed to sunlight, results in the formation of the grains of starch found in these corpuscles.