256 The CarhoIiif^Jnifrs ,,f the Mfon/ohf Leaf 



that glucose precedes starch in the process of assimilation, the starch 

 being formed from the glucose when the concentration of the latter 

 exceeds a certain maximum which differs in different plants. Taking 

 into account Baeyer's hypothesis, advanced in 1870, it was possible 

 to regard the glucose as formed by the polymerisation of formaldehyde, 

 CHgO, which was itself produced directly by the reduction of carbonic 

 acid in the leaf imder the influence of light and chlorophyll. 



Some such view as this appears to have been generally held down 

 to 1893, although Arthur Meyer [188-5] emphasised the fact that the 

 formation of starch in plants was by no means imiversal^; many 

 monocotyledons, partic\ilarly the Liliacece, form very little starch in 

 their leaves, some none at all (e.g. Allium cepn, Scillu maritium). An 

 important point estabUshed by Meyer was that leaves of plants which 

 store starcji abundantly contain comparatively little sugar, whilst 

 plants like Gentiana liUea, Allium and Asphodel, which form little or 

 no starch in the leaf, accumiilate large stores of soluble reducing sub- 

 stances, probably glucose. When starch is absent Meyer considers that 

 its place may be taken as a reserve substance by other sparingly soluble 

 carbohydrates, such as inulin {sinislrin), which he isolated from the 

 leaves of the Yucca. In 1886 Meyer [1886] made the interesting 

 observation that almost all leaves which are capable of forming starch at 

 all, produce it abundantly from a 10 per cent, solution of laevulose and 

 a relatively small number from dextrose. 



The important work of Brown and Morris in 1893 ma}^ be regarded 

 as marking the beginning of a new period in the study of the physiology 

 of the leaf. Up to this date, the chemical methods used had been 

 largely qualitative or very roughly quantitative ; little attempt had been 

 made to discriminate between the reducing sugars of the leaf, which 

 were universally regarded as glucose (dextrose). The possibility of 

 laevulose (fructose) being present or a mixture of laevulose and dextrose 

 (invert sugar) had hardly been raised, whilst maltose had not been 

 suggested as a possible leaf carbohydrate, although it was known that 

 this sugar was formed from starch by the action of diastatic enzymes. 

 Cane sugar, although kn(nvn to be widely present, had not been taken 

 into account as a possible direct product of photosynthesis (except by 

 A. Girard [1884] in an important paper referred to later) ; it was generally 

 regarded as a reserve carbohydrate formed as a secondary jiroduct 

 from the simpler sugars. 



' Bochin [1866] had found that the chloroplasts of Allium species, Galanthui, Hyacin- 

 thiis, Ornithogalum and Irin germiiiiica normally do not form starch. 



