1893. PLANT NUTRITION. 21 



green parts of plants as an assimilatory process, dates from the begin- 

 ning of the century ; the essential importance of light was also recog- 

 nised by the early workers, but it is to Sachs that we owe the discovery 

 of the connection between this carbon-assimilation and the production 

 of a definite substance — starch — in the chloroplasts of the leaf. 



The occurrence of starch granules in the chloroplasts was made 

 known by von Mohl in 1837, but Sachs, in 1862, observed that it 

 was induced by the same conditions as those known to be necessary 

 for the decomposition of carbon dioxide by plants, namely, exposure 

 to light, and his memoirs of 1862-4 deal with the influence of light on 

 starch production in the chloroplasts. He shows that where carbon 

 dioxide is being absorbed there is a constant production of starch 

 when the plant is exposed to sufficiently bright daylight, a subsequent 

 disappearance and removal of this starch from the leaf at night, and 

 for the first time explains the connection between carbon-assimilation 

 and starch production by the leaves. Godlewski, in 1873, strengthened 

 Sachs' conclusions by the demonstration that no starch-grains are 

 formed in the leaf-chloroplasts when carbon dioxide is missing from 

 the surrounding atmosphere, even when the plant is exposed to bright 

 light. 



When decomposition of carbon dioxide and exhalation of oxygen 

 is going on, starch in nearly all cases makes its appearance in the 

 green chloroplasts of the leaf, yet Sachs' work does not show that 

 starch is the immediate and direct product of this assimilation, 

 although the fact that the volume of oxygen given off equals the 

 volume of carbon dioxide absorbed was at first taken as a support 

 for this hypothesis. That a complicated compound, such as starch, 

 should be actually formed directly from carbon dioxide and water 

 was highly improbable, and since Sachs' first papers there has 

 accumulated a large amount of evidence pointing to the formation of 

 some much simpler carbohydrates, which are transformed into starch 

 by the chloroplast, probably only when they are in excess of the 

 immediate requirements of the cell for its own nourishment and 

 respiration — that starch is, in fact, always a reserve material and is 

 only the first visible product in a large series beginning with carbon 

 dioxide and water. 



Bohm (1877) showed that starch can be formed in the chloroplasts 

 from materials which have been elaborated elsewhere than in the cell 

 in which starch-production is going on, and Schimper (1880) paved 

 the way for further research in this direction by his discovery of the 

 starch-forming corpuscles or amyloplasts in parts of plants not green 

 (tubers, stems, &c.) and their morphological identity with chloroplasts. 

 These bodies are colourless and are concerned with the formation of 

 starch out of ready-formed sugars supplied to them, and can be 

 changed into chloroplasts by exposure to light in many cases. 



Schimper's conclusion that the chloroplasts of the leaf parenchyma 

 have not the power to produce starch except by assimilation from 



