MBTABOLISM 275 
bye-products of metabolism is not at all well defined. In 
many cases substances are included in the latter category 
because nothing is known as to their function, and the 
classification can therefore be regarded only as provisional. 
In many cases it cannot yet be determined whether 
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 
Fig. 124£,—GianpuLar Hairs rrom 
Primula sinensis. 
5 é " Vie. 125.—GLanpuLvR Haris rrom 
u, young hair; b, hair showing sccrc- 
tion formed in the cell-wall of the tHe Hop. 
terminal cell ; c, hair after discharge A, young hair; B, mature hair ; 
of the secretion. 8.¢., secretion under the cuticle. 
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 
(tig. 124), and the more complex one of the Hop (fig. 125), 
have long been known to form their resins in this way. It 
seems probable that we must now regard the regin-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 
18 * 
