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. Recent 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) 
Fic. 124.—Gbanpunan Hairs rRom 
Primula sinensis. 
w, young hair; 5, hair showing secre- Fie. 125.—GuanpuLaR Harps FROM 
tion formed in the cell-wall of the tHE Hor. 
terminal cell; c¢, hair after dis- a, young hair; B, mature hair; 
charge of the secretion. 8.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- 
