METABOLISM 



269 



particular substances are formed by the direct decomposi- 

 tion of protoplasm, or by subsequent changes in the primary 

 products of such decomposition. Till quite recently the 

 formation of resin and allied bodies in the resin passages 

 of the Conifers and in many glandular hairs was con- 

 sidered a true secretion, the aromatic substances being held 

 to arise in the cells. Kecent investigations tend to show that 

 this is not the mode of their origin at all, but that these 

 substances are formed by a peculiar process of degradation 

 of the cell-wall. The glandular hairs of Primula sinensis 

 (fig. 124), and the more complex one of the Hop (fig. 125) 



FIG. 124. GLANDULAR HAIRS FROM 

 Primula sinensis. 



a, young hair ; &,hair showing secre- 

 tion formed in the cell-wall of the 

 terminal cell ; c, hair after dis- 

 charge of the secretion. 



FIG. 125. GLANDULAR HAIRS FROM 

 THE HOP. 



*A, young hair; B, mature hair; 

 s.c, secretion under the cuticle. 



have long been known to form their resins in this way. It 

 seems probable that we must now regard the resin-secret- 

 ing organs of the Conifers as comparable with these. 



The bye-products of metabolism are too numerous to 

 be discussed in detail in the present treatise. Though 

 they seem to be quite subordinate to the main products we 

 have noticed, and to be formed indeed by decompositions 

 which take place during the construction of the latter, we 

 should not be warranted in ignoring their possible utility 

 to the plant, nor the probability that many of them may 

 be of nutritive value. We have seen that in the decom- 

 position of amygdalin by its appropriate enzyme emulsin, 

 besides the undoubtedly nutritive sugar there is a produc- 



