234 Transactions of the Royal Microscopical Society. 
from those which in the period of their maturity are green. This 
change is often noticed in the commoner sorts of plants in almost 
every stage, though in the case of evergreens it is more or less 
confined to a few. Take one instance of it, the arbutus, where 
the leaves undergo several distinct changes of colour from the 
time that they are newly developed upon the young shoots, till 
they become either dying or dead. 
Looking cursorily or rather without the eye of a micro- 
spectroscopist at the young newly-formed leaves of arbutus, the 
difference in shade as to colour, &c., is so very slight to those 
which are older and more mature, that one might be almost 
inclined to doubt whether or not the selection of this example was 
not rather in opposition to the views lately laid down. However, 
look at Figs. 8 and 9 where the difference mainly and only due 
to the action of light and temperature, &c., is clearly seen : thus 
we find that these young leaves do to a certain extent contain a 
quantity of those essential elements which constitute their life in 
the shape of a highly wax-like or other secretiou, for it is 
particularly noticeable how they are, as it seems, coated or rather 
thoroughly impregnated with this greasy material, which, as the 
accumulative action both of light and temperature takes place, 
forms both a varnish for the sake of protection and acts probably 
also as the chief substance from which the other parts are hereafter 
formed. They moreover seem to need the action of light to a 
much greater extent than laurel and holly, as those leaves which 
grow within the shrub, and are therefore more or less shaded, 
wither quicker than those which are exposed. 
The peculiarity, however, which I more especially wish to call 
your attention to is that these deep green leaves, when their functions 
with regard to the plant have been thoroughly carried out, that is 
to say, when an equilibrium of their mature state has been esta- 
blished, and they have arrived at the stage of the third order, or 
the period of descension, they gradually, and indeed somewhat 
quickly, assume a pink-re 1 hue, which is evidently only another 
product formed by the action of light ; for when at the commence- 
ment of this state they are shaded as before described, the green 
tint is preserved till they have gone too far for subsequent recovery 
when they will be found to have withered up entirely, whereas 
while allowed to be exposed, the pink colour which will be seen 
on turning the leaf over to be only on the surface, is but one step 
in the ladder reached, for we find the red resolving into yellow, 
and the yellow to total death, which is evinced by the appearance 
of dark brown spots on the epidermis, which eventually impregnate 
the whole leaf. If, therefore, we deduce that the red and yellow 
substances are formed by the direct action of light, and are there- 
fore due to some direct change having taken place in the pre- 
