228 Transactions of the Royal Microscopical Society . 
the plant. It is very remarkable to observe the number of curious 
facts presented to us in relation to this subject, for we find light 
acting, 1st, as a constructive, and 2nd, as a destructive agent ; at 
the same time, who knows, says Mr. Sorby, but that these meta- 
morphoses are not of the most vital importance to the life of a 
plant, I mean the decomposing action on the colouring matter of 
leaves ? The red substances formed from chlorophyll by the action 
of light are hereafter decomposed by it ; and if we suppose that 
both chlorophyll and other colouring matters, such as the yellow, 
red, blue, and brown products of its decomposition are, besides the 
independent colours, in some way formed by the action of light, 
from other constituents of the plant, and are hereafter decomposed 
by the same agency, we can far more easily explain many re- 
markable facts, such as the various changes in shade for instance, 
from the earliest forms to the deeper or lighter colours, either of 
their mature or decaying state. 
Thus we gather from these observers, experiments how leaves 
gradually become less and less fit for this process of transpiration. 
Sennebier found that when all other things are equal, the transpi- 
ration is much greater in May than in September, hence the 
reason surely that some leaves are renewed annually. Their 
organs, moreover, become gradually unfit for performing those 
especial functions that are so necessary to them, and therefore, as 
a matter of fact, it is advisable that they should be renewed. 
Those trees which retain their leaves during winter months, have 
been shown to transpire less than others ; it consequently takes a 
longer period for them to carry out those several functions which 
nature has allotted to them. It is, however, well known that 
evergreen plants do also renew their leaves. The delicate shades 
of yellow, rather than green, which characterise the young newly- 
developed leaves of the laurel tribe are very beautiful, and the 
spectrum produced by a solution in alcohol tends to show how 
much the constructive energy of light and other acting agents do 
to them ere they become the dark forms which adorn our garden 
hedges. A waxy substance pervades to a considerable extent the 
leaves of all evergreen plants. It exists both as an external 
coating to the epidermis, and so doubtlessly protects the leaves from 
the action of wet, and other changes in the weather, as well as 
pervading the whole structure. It is curious also to note how it 
protects the chlorophyll from the action of solvents, those such as 
ether taking hardly any effect, while alcohol extracts it all in a 
remarkably short space of time; our spectrum in Fig. 2 is from a 
solution of these leaves prepared in alcohol, while Fig. 3 is from 
one made up in ether. The difference is too striking to require 
any comment ; suffice it to say that those macerated in alcohol 
were reduced to the yellow state which is so often seen in the case 
of natural decadence. It seems to me as though this waxy com- 
