Page:Proceedings of the Royal Society of London Vol 1.djvu/194

Rh posited in the alburnum sometimes remains unemployed during several successive years; he therefore cut off, in the winter, all the branches of a large and very old pear-tree, at a small distance from the trunk, and pared off, at the same time, all the lifeless external bark. No marks of vegetation appeared till the beginning of July following, when numerous buds and leaves, of large size, appeared: and in autumn every part was covered with very vigorous shoots. The number of leaves appeared to Mr. Knight to exceed very much the whole of those the tree had borne in the three preceding years.

Mr. Knight says that he has repeated, with success, the experiments of Bonnet and Du Hamel, and that he is in possession of many other facts which, like those experiments, tend to prove that seedling trees depend, at ﬁrst, entirely on the nutriment afforded by the co- tyledons ; and that they are greatly injured, and often killed, by being put to vegetate in rich mould. He thinks there is very decisive evi- dence that bulbous and tuberous-rooted plants contain within them- selves the matter which subsequently composes their leaves ; also that it appears extremely probable, that the blossoms of trees receive their nutriment from the albumum, particularly as the blossoms of many plants precede their leaves.

Mr. Knight also thinks the existence of a vegetable circulation, though denied by many eminent naturalists, must be admitted. He supposes that when a seed is placed in a proper situation for vege- tation, water is absorbed by the cotyledons, and a young radicle is emitted. This increases in length, by the addition of new matter to its apex, not by any general distension of its vessels or ﬁbres; which new matter appears, from the experiments of Bonnet andDu Hamel, to deseend from the cotyledons. The ﬁrst motion, therefore, of the ﬂuids is downwards, towards the point of the root; and the vessels which carry those ﬂuids are similar to those which are subsequently found in- the bark. In support of this opinion, he mentions some ob- servations he has made on the progressive changes which take place in the radicle of the horse-chestnut. From these it appears, that when the roots were considerably elongated, and not till then, albur- nous tubes were formed, and that as soon as these tubes had acquired a suﬁicient degree of ﬁrmness, they appeared to begin their oﬂice of carrying up the aqueous sap: at which time, and not sooner, the leaves of the plumula expanded. When the leaf has attained its proper growth, it seems to perform precisely the ofﬁce of the cotya ledon, being fed by the albumons tubes and central vessels ; and the true sap is discharged from the leaf, as it was previously from the cotyledon, into the vessels of the bark. Here one part of it produces the new layer of wood (or new epidermis when that is to be formed), and the remaining part enters the pores of the wood already formed, and mixes with the ascending aqueous sap.

The author thinks it probable that the true sap undergoes a considerable change on its mixture with the ascending aqueous sap, as in the sycamore; it was found to become more sensibly sweet in its progress in the root, in the spring, although he could never detect