“It is also supposed that the vessels of the petiole gradually become woody, and incapable of freely transmitting the sap; it therefore stagnates, the vessels become overloaded, and the parts which connect the stem and the leaf crack at the insertion of the petiole. The vessels being thus interrupted, the leaf is no longer supplied with proper nourishment; it loses its elasticity, and becomes dry and brittle; and the least shock, whether the effect of frost or of wind, detaches it.
“Another opinion,” added my uncle, “is, that the fibres of the leaf-stalk are not a simple continuation of those of the twig or branch, but that they both terminate at that point from which the leaf falls; being only connected by a kind of adhesive substance, which dries up when the sap ceases to rise. This point of separation you may easily perceive,” said he, “like a cicatrice, in the form of a ring; and the same appearance of a natural separation is to be seen in the peduncles of flowers, which seem also to be attached by a sort of vegetable solder to the stem.”
“But, uncle, why then do not leaves fall much sooner, if they are so slightly attached to the stem?”
“Because this adhesive substance is a strong cement, as long as it is supplied by the vegetable juices. If you attempt to remove the stalk elsewhere than at that point where it is united, the fibres are lacerated; and this proves that the separation had been prepared for at that one point, by some peculiar organization which acts independently of frost or rain, or other external causes.”
My uncle then shewed me the ring which marks the point of separation. It is most easily seen in autumn, he says; it is double in the orange, and in the berberry he shewed me that it is above the point of contact between the leaf and branch, so that after the fall of the leaf, the rudiment of the foot-stalk remains to preserve the bud. He took the trouble of pointing out a little triangle of thorns behind the young bud, which seems to be another beautiful contrivance of nature for its protection in that bush. We then observed this point of separation in other trees whose leaves were not at all gone, and he told me that it is very strongly marked in the horse-chesnut with five small dots.
I begged my uncle to tell me what I should particularly pay attention to in the course of our observations.
He said that as it has never yet been ascertained whether the leaves spring from the wood or from the bark, he would advise us not merely to observe the progress of the buds, but to take every means of tracing their connection with the interior. We may examine with his microscope all sorts of twigs, to see whether the vessels of the central part of the wood extend to the leaf-stalk; and he suggests that we should very carefully observe the difference of structure in evergreen leaves, and in those which perish in the autumn. He recommends us to lose no further time in beginning our task on the few remaining leaves, in order that we may see in spring whether evergreens shed their leaves in the same manner; and we are also to ascertain when their buds are actually formed. “Above all,” he says, “I advise you to take nothing for granted—examine every thing with your own eyes, and learn facts.”
I shall like this employment very much, and Mary, Frederick, and myself have agreed to work in concert. Both my uncle and aunt encourage us; they say it will afford a large field for very entertaining experiments, and they think that inquiries of this sort are highly useful to young people.
23d.—The fields which were ploughed and sown with wheat not above two months ago, are now of a beautiful green; how hardy it must be, to withstand the severe weather, which I am told may soon be expected! My uncle says, that wheat grows in every variety of climate, except in regions of extreme cold.
It has not been ascertained of what country wheat was a native, and it is certainly a very remarkable fact, that, though cultivated so generally, no wild plants of those species that are used in agriculture have been found, though one of our late travellers imagined that he found it in the mountains of Thibet.