20 NATURAL SCIENCE. July, 



He also established the absolute necessity of carbon dioxide for plant 

 growth, pointed out its injurious nature when present in more than 

 comparatively small amounts, and, finally, made the most important 

 discovery that, while the decomposition of carbon dioxide and 

 exhalation of oxygen are going on in sunlight, the plant is increasing 

 considerably in dry weight. This increase is, moreover, greater than 

 can be accounted for by the fixation of carbon alone, the excess 

 being due to the simultaneous fixation of the elements of water — 

 hydrogen and oxygen — supplied from the ground through the roots. 



With all this accumulated knowledge, it is somewhat difficult to 

 understand why chemists and plant physiologists did not at once 

 begin to inquire into the nature and composition of the product first 

 formed in green leaves as a result of the processes described. The 

 whole question of carbon-assimilation by leaves had been well opened, 

 but the experiments were regarded by many of the chief workers in 

 this field as having not even the remotest connection with nutrition of 

 plants, and were consequently cast aside, or at least neglected and 

 almost forgotten. This was largely due to preconceived notions 

 respecting plant food, and the state of chemical knowledge at the 

 time was such as to check progress. The early Aristotelian teaching — 

 that the sole food-substance of all plants was " humus " — prevailed, 

 and chemists were almost compelled to adopt this view because of 

 their conceptions regarding the formation of organic compounds. 

 Animal, vegetable, and mineral chemistry had been found to be an 

 inconsistent classification, as some compounds belonging to the first 

 two groups were found to be identical ; but a more disastrous division 

 into organic and inorganic chemistry took place, the latter dealing 

 with elements under the influence of physical and chemical forces as 

 at present understood, whereas into the former was introduced a " vital 

 force" which helped to get over difficulties otherwise not amenable 

 to treatment, and at the same time made attempt at a reasonable 

 explanation somewhat superfluous. It was dogmatically asserted that 

 substances formed under the influence of the vital principle could not be 

 produced artificially, and, unfortunately, Wohler's synthesis of urea in 

 1828 was incomplete, and remained for a long time the only case 

 tending to support an opposite opinion. The existence of such a 

 force, which was even capable of the creation of plant-ash out of 

 nothing, made careful observation unnecessary, and it was not until 

 after the complete destruction of the humus-theory of plant nutrition 

 by Liebig in 1840, and Boussingault a little later, that the mind was 

 free to look at the significance of the work of the early experimenters. 



From this period down to the present time the assimilation or 

 preparation by plants of organic compounds from simple substances, 

 such as carbon dioxide, water, and ammonia, supplied to them from 

 the surrounding atmosphere and soil, has been the subject of 

 numerous and elaborate investigations. The recognition of the 

 decomposition of carbon dioxide with elimination of oxygen by the 



