The hydrolysed extract was neutralised and fermented. In the early stages of growth the furfuroids were completely fermented, i.e. disappeared in the fermentation. In the later stages this proportion fell to 50 p.ct. In the earlier stages, moreover, the normal hexose constituents of the permanent tissue were hydrolysed in large proportion by the acid, whereas in the matured straw the hydrolysis is chiefly confined to the furfuroids. In the early stages also the permanent tissue yields an extract with relatively low cupric reduction, showing that the carbohydrates are dissolved by the acid in a more complex molecular condition.
These observations confirm the view that the furfuroids take origin in a hexose-pentose series of transformations. The proportion of furfuroid groups to total carbohydrates varies but little, viz. from 1/3 in the early stages to a maximum of 1/4 at the flowering period. At this period the differentiation of the groups begins to be marked.
Taking all the facts of (1) and (2), they are not inconsistent with the hypothesis of an internal transformation of a hexose to a pentose-monoformal. Such a change of position and function of oxygen from OH to CO within the group —CH.OH— is a species of internal oxidation which reverses the reduction of formaldehyde groups in synthesising to sugars, and appears therefore of probable occurrence.
These constitutional problems are followed up in (3) by the indirect method of differentiating the relationships of these furfuroids to yeast fermentation, from those of the pentoses. Straw and esparto celluloses are subjected to the processes of acid hydrolysis, and the neutralised extracts fermented. With high furfural numbers indicating that the furfuroids are the chief constituents of the extract, there is an active fermentation with production of alcohol. The cupric reduction falls in greater ratio to the original (unfermented) than the furfural. Observations on the pure pentoses—xylose and arabinose added to dextrose solutions, and then exposed to yeast action—show that in a vigorous fermentation not unduly prolonged the pentoses are unaffected, but that they do come within the influence of the yeast-cell when the latter is in a less vigorous condition, and when the hexoses are not present in relatively large proportion.
(4) The observations on the growing plant were resumed with the view of artificially increasing the differentiation of the two main groups of carbohydrates. From a portion of a barley crop the inflorescence was removed as soon as it appeared. The crop was allowed to mature, and a full comparison instituted between the products of normal and abnormal growth. With a considerable difference in 'permanent tissue' (13 p.ct. less) and a still greater defect in cellulose (24 p.ct.), the constants for the furfuroids in relation to total carbohydrates were unaffected by the arrested development. This was also true of the behaviour of the hydrolysed extracts (acid processes) to yeast fermentation.
(5) The extract obtained from the brewers' grains by the process described in (2) was investigated in relation to animal digestion. It has been now generally established that the furfuroids as constituents of fodder plants are digested and assimilated in large proportion in passing through animal digestive tracts, and in this respect behave differently from the pentoses. The furfuroids being obtained, as described, in a fully hydrolysed condition (monoses) the digestion problem presented itself in a new aspect, and was therefore attacked.
The result of the comparative feeding experiments upon rabbits was to show that in this previously hydrolysed form the furfuroids are almost entirely digested and assimilated, no pentoses, moreover, appearing in the urine.
Generally we may sum up the present solution of the problem of the relationship of the furfuroids to plant assimilation and growth as follows:—The pentoses are not produced as such in the process of assimilation; but furfural-yielding carbohydrates are produced directly and in approximately constant ratio to the total carbohydrates; they are mainly located in the permanent tissue; in the secondary changes of dehydration, &c., accompanying maturation they undergo such differentiation that they become readily separable by processes of acid hydrolysis from the more resistant normal celluloses; but in relation to alkaline treatments they maintain their intimate union with the latter. They are finally converted into pentoses by artificial treatments, and into pentosanes in the plant, with loss of 1 C atom in an oxidised form. The mechanism of this transformation of hexoses into pentoses is not cleared up. It is independent of external conditions, e.g. fertilisation and atmospheric oxidations, and is probably therefore a process of internal rearrangement of the character of an oxidation.