17* 



posited in the alburnum sometimes remains unemployed during se- 

 veral 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 experi- 

 ments 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 alburnum, 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 and Du Hamel, 

 to descend 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 sufficient degree of firmness, they appeared to begin their office of 

 carrying up the aqueous sap ; at which tune, 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 coty- 

 ledon, being fed by the alburnous 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 con- 

 siderable 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 



