158 METABOLISM 



rise to mannose and galactose in quantity but only to a little dextrose, on 

 treatment with dilute acids, but only if the acids be dilute. SCHULZE terms 

 them hemicelluloses in contrast to the true celluloses, which are capable of 

 hydrolysis only by the action of concentrated acids. These latter can ob- 

 viously be dissolved by the plant with great difficulty and never take any 

 further part in metabolism. 



The celluloses are perhaps better characterized by their behaviour in the 

 presence of enzymes than in the presence of acids. Not only is this of more 

 importance biologically, but it gives us also a clearer insight into their chemical 

 nature. It is well known that the hemicelluloses are dissolved by enzymes in 

 seeds, although we have no precise knowledge of these enzymes. BROWN and 

 MORRIS (1890), from germinating barley, and NEWCOMBE (1889), from the 

 cotyledons of lupins, as well as from the endosperm and the cotyledons of 

 Phoenix, by the same method used in the extraction of diastase, have obtained 

 a soluble enzyme which dissolves the cell-walls of the endosperm of barley 

 rapidly, and more slowly the reserve cellulose of lupins. Although this power 

 of dissolving the cell-wall has been attributed for several reasons to diastase, 

 which as a matter of fact is never absent from such extracts, NEWCOMBE was 

 able to prove conclusively the presence of a special enzyme, cytase. Although 

 it has not been as yet possible to separate this enzyme from diastase, still NEW- 

 COMBE'S conclusion would appear fully justified because the amylolytic and 

 cytolytic capabilities of the extract are not at all proportional to each other, 

 the extract of lupins and of Phoenix having a very vigorous action on cellulose 

 and a feeble action on starch, while malt extract acts conversely. 



The distribution of the cytases has not as yet been fully determined, but 

 it may be assumed that they occur wherever cellulose requires to be dissolved. 

 That they frequently cannot be recognized may be accounted for by the fact 

 that they are present only in very small quantities ; indeed we find that the 

 endosperm of palms, for example, takes a much longer time to dissolve than that 

 of the Gramineae. [HERISSEY (1903) has shown that cytases are widely dis- 

 tributed, and that there are many types of them, each with its own specific 

 activity.] 



In addition to starch and cellulose a third non-nitrogenous substance 

 occurs in seeds, viz. fatty oil. Fatty oils occur not in seeds alone but in all 

 cells ; indeed it is very doubtful whether there is such a thing as protoplasm 

 free from fat. It is a normal component of the chloroplasts of certain 

 plants, and was in such cases for long believed to replace starch as the final pro- 

 duct in the process of carbon assimilation. More exact investigations (HoLLE, 

 1877, GODLEWSKI, 1877) do not confirm this view. It seems much more likely 

 that the oil globules occurring in chloroplasts are not to be considered as pro- 

 ducts of assimilation, at least in the higher plants (although in Vaucheria 

 FLEISSIG (1900) holds that fat does take the place of starch), and that they 

 are not capable of further conversion. It is for that reason we have made no 

 mention of fats in the preceding pages. In seeds, fats are undoubtedly reserves, 

 and in many seeds they form the chief part of them, as may be seen from the 

 following table : 



Almond . . . 5.39 24.18 53-68 7-23 6-56 2-96 



Hazel-nut . . 3.77 15-62 66-47 9 >O 3 3 <a8 I>8 3 



Poppy-seed . . 5.79 14-09 47-69 18-74 5-76 7-93 



Coco-nut . . . 5-81 8-88 67.00 12-44 4-06 1.81 



We must treat of the fats in this section, although we cannot enter into 

 a discussion of their mode of origin at the present moment. 



The fats are really glycerine esters of various fatty acids, belonging to one 

 or other of three series. The first series has a composition represented by the 



