PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
91 
Such, then, is a history of the work with which the leaf 
has been busy all summer ; let us see now what becomes 
of it in autumn. All through the season the sap has been 
coming into it to be made fit for the nourishment of the 
plant, but by this process not only the sap but the leaf 
itself has been constantly undergoing a change. The sap 
comes into it charged with mineral matters, which were 
dissolved in the water drawn up by the rootlets from the 
soil. But when it is in the leaf it suffers, as we have seen, 
a certain amount of evaporation, and it is therefore obliged 
to lose a portion of its mineral contents, which is then de¬ 
posited in the vessels of the leaf-stalk and veins. Thus it 
comes about that, by the end of the season of activity, 
these vessels become entirely choked with this deposit, and 
from this two results follow. On the one hand the up¬ 
ward flow of the sap is gradually arrested, and, on the 
other, the leaf fades and dies tor lack of moisture and 
nourishment. It is at this point of its history, when its 
work is almost finished, and just before it falls from the 
tree, that it assumes its autumnal colouring. The actual 
chemical change which then takes place is due to a variety 
of causes which are not yet clearly understood, but this we 
know, that all the varied shades of red, crimson, yellow, 
and brown are produced by the same substance which 
made the leaf green in summer—namely, chlorophyll, 
though in a slightly altered form. The change which has 
taken place in its chemical constitution is mainly due to 
the loss of protoplasm, starch, and other substances which 
have been withdrawn from the cells of the leaf into the 
bark and branches of the tree. Two circumstances oc¬ 
curred to me while watching the progress of the tints last 
autumn, as tending to confirm this hypothesis. The first 
was that in each leaf the first parts to change colour were 
those farthest from the veins ; while, in the second place, 
the fi rst leaves on the tree to become tinted were always those 
at the summit and at the extremities of the larger branches. 
These, it will be seen, would be the first parts of the leaf 
and of the tree to be affected by such a withdrawal. In 
connection with this Mr Grant Allen remarks in his de¬ 
lightful sketches, entitled Vignettes from Nature, that 
some of the autumn tints “Are actually present in th e 
green leaf itself, though completely masked during the 
period of vigour by the preponderance of the natural pig¬ 
ment which owes its colour to the due admixture of them 
all.” In this withdrawal we may observe a most beautiful 
provision of Nature for the sustenance of the plant, for if 
these stores, which the leaf has been at such pains to 
manufacture, were not thus withdrawn, they would be 
sacrificed and wasted when the leaf falls. As it is, how¬ 
ever, they are carefully stored up in the tough tissues of 
the tree for its use during the winter months, and for the 
nourishment of the young buds in spring before they are 
exposed to the light, and so enabled to obtain air-food for 
themselves. 
At the same time that the leaf is assuming its autumn 
colouring, another change is taking place, also preparatory 
to its ultimate separation from the branch. If you examine 
the scar which is left on the latter after the'leaf has fallen 
from it, you will observe that it is not ragged, but as 
smooth as if the leaf-stalk had been cut off with a sharp 
knife; and farther, this scar, even immediately after the 
leaf has been shed, is perfectly dry, showing that no sap 
or moisture is escaping from the ends of the severed 
vessels. The reason of this is that with the approach of 
autumn the set of cells which form the junction of the 
leaf-stalk with the branch gradually shrivel up, so that 
when the work of the leaf is finished, and after it has de¬ 
livered up all its stores of nourishment to the parent tree, 
it gradually fades and dies, and then gently falls from the 
branch, either by its own weight, or the touch of the 
autumn breeze. In this way Nature anticipates the fall 
of the leaf, and makes provision that no particle of plant- 
food should be lost by bleeding, such as would result were 
the leaf left to be torn off by the wind. The precise 
nature of the change which takes place in these junction 
cells is still a matter of dispute among botanists. 
Before leaving the autumn leaf, we may point out the 
end which it has to fulfil in the economy of Nature after it 
has fallen to the ground. You are all aware what valu¬ 
able mould is produced, by the decay of fallen leaves. 
The reason of this is simply the large amount of mineral 
matter contained in their tissues, and which, as we have 
seen, was the cause of their death. In this we have a 
further illustration of how none of Nature’s products are 
wasted, for part of this mineral matter which is necessary 
in one stage of the plant’s growth, but would be injurious 
in another stage, is arrested by the leaves and by them 
returned to the soil to assist in the fresh growth which is 
to take place next spring. 
Such are some of the thoughts which are suggested by 
the autumn woods, when, in the words of the author of 
the Christian Year, we 
Watch the calm leaves float. 
Each to his rest, beneath their parent shade. 
