412 PHYSIOLOGY 
that all fermentations are effected by enzymes, as some are known to 
be, and that the formation of these enzymes is- not so much a matter of 
advantage to the organism as an inevitable result of the conditions under 
which it develops. If this be true, to seek for explanation through ad- 
vantage is a fruitless quest. 
3. WASTE PRODUCTS AND ASH 
Wastes not useless. In the course of the many and varied chemical 
changes which take place in plants, there arise, especially in consequence 
of the destructive metabolism, a great number of compounds which are 
not usable for the building of new parts, and are not again drawn into 
the metabolism. Some of these are nevertheless of considerable service 
to the plant, and in varied ways; as, for example, in protecting it from 
predatory animals by disagreeable tastes or odors, in covering wounds 
by gummy or resinous exudations, in attracting by color or odor insects 
which effect pollination, etc. In spite of the usefulness of some of them, 
these substances are often called waste products, and this word may 
well be retained instead of the more technical term, aplastic products, 
which has been applied to them. For in every household there are like 
products, properly " waste," so far as the direct economy is concerned, 
some of which may nevertheless be collaterally serviceable. 
Number. Of the reactions by which these waste products are pro- 
duced, not much is known, and they need not be considered at all here. 
The number of the products is very great, and it is possible to name 
only a few of the more important groups and examples of them. An im- 
pression of their number may be gained from the fact that in a recent 
work on plant chemistry more than 4000 are mentioned, and the book 
does not pretend to enumerate all known substances. Thus there are 
over 200 known alkaloids, and a single firm lists some 200 essential oils 
of commercial value. Yet the knowledge of the chemistry of plants is 
very incomplete and lags far behind that of animals. 
No true excretion. Almost all of the wastes accumulate in the tis- 
sues, for actual excretion by plants is very imperfect. Except for those 
which are got rid of in the fragments of bark, roots, twigs, and leaves 
that are shed, and the relatively minute quantities that are secreted by 
surface glands, or diffuse out into the water from roots and other im- 
mersed parts, there is no provision for doing more than storing these 
substances in some out-of-the-way place. In no case is there any ar- 
