96 ACCOUNT OF SOME EXPERIMENTS ON THE 



ignorant of the channels through which the additional matter is con- 

 veyed to it. 



I will now take the liberty of stating a few of the conclusions that I 

 have ventured to draw from the foregoing, and many similar experi- 

 ments. As I have not been able to find the spiral tubes anywhere, 

 except immediately surrounding the medulla in different parts, in the 

 seed, and in the leaf, and as they everywhere terminate at short 

 distances, I conclude that the sap is not raised by their agency ; 

 nor by the central vessels, to which they are appendages : for these 

 extend no greater length downwards than the spiral tubes, and ter- 

 minate with them at the external surface of that annual layer of 

 wood to which they belong ; and they have not any apparent communi- 

 cation with the similar vessels of the succeeding year. In the lower 

 parts of hollow trees they must long have ceased to exist at all : and, in 

 all trees, except very young ones, they are (as it were) ossified within 

 the heart wood ; and those in the annual shoots and buds are often a 

 hundred- and- fifty feet distant from the roots, from which they are sup- 

 posed to raise the sap. 



The common tubes of the alburnum, (which do not appear to me to 

 have been properly distinguished from the central vessels by the authors 

 that I have read,) extend from the points of the annual shoots to the extre- 

 mities of the roots ; and up these tubes the sap most certainly ascends, 

 impelled, I believe, by the agency of the silver grain. At the base of the 

 buds, and in the soft and succulent part of the annual shoot, the albur- 

 num, with the silver grain, ceases to act and to exist ; and here, I believe, 

 commences the action of the central vessels, with their appendages, the 

 spiral tubes. By these the sap is carried into the leaves, and exposed to 

 the air and light ; and here it seems to acquire (by what means I shall 

 not attempt to decide) the power to generate the various inflammable 

 substances that are found in the plant. It appears to be then brought 

 back again, through the vessels of the leaf stalk, to the bark, and by that 

 to be conveyed to every part of the tree, to add new matter, and to com- 

 pose its various organs for the succeeding season. When I have inten- 

 tionally shaded the leaves, I have found that the quantity of alburnum 

 deposited has been extremely small. 



In speaking of the circulation within the apple and pear, I wish to 

 express myself with much less decision, as I have not seen the effects of 

 taking up any of those vessels into which the coloured infusions did not 

 enter. The internal organization of the leaf, and of the wood, of those 

 trees which have a central medulla, seems to admit but of little variation, 



