300 On -the Origin" and Office of the Alburnum of Tree*. 



and those of wlrch I have given an account in former me- 

 moirs, afford sufficient evidence that the bark deposits the 

 albumous matter. If the succulent shoot of a horse ches- 

 nut, or other tree, be examined, at successive periods in the 

 spring, it will be seen that the alburnum is deposited, and 

 its tubes arranged, in ridges beneath the cortical vessels ; 

 and the number of these ridges, at the base of each leaf, will 

 be found to correspond accurately with the number of aper- 

 tures through which the vessels pass from the leaf-stalks 

 into the interior bark, the albumous matter being apparently 

 deposited (as I have endeavoured to prove in former me- 

 moirs) by a fluid which descends from the leaves, and sub- 

 sequently secretes through the bark*. I shall therefore ven- 

 ture to conclude that it is thus deposited, and shall proceed 

 to inquire into the origin and office of the albumous tubes. 



The position and direction of these tubes have induced al- 

 most all naturalists to consider them as the passages through 

 which the sap ascends ; and at their first formation, when 

 the substance which surrounds them is still soft and succu- 

 lent, they are always filled with the fluid, which has appa- 

 rently secreted from the bark. They appear to be formed in 

 the soft cellular mass, which becomes the future alburnum, 

 as receptacles of this fluid, to which they may either afford 

 a passage upwards, or simply retain it as reservoirs, till ab- 

 sorbed, and carried off, by the surrounding cellular sub-? 

 stance. The former supposition is, at first view, the most 

 probable; but the latter is much more consistent with the 

 circumstances that I shall proceed to state. 



Many different hypotheses have been offered by naturalists 

 to account for the force with which the sap ascends in the 

 spring; of these hypotheses t\vo only appear in any degree 

 adequate to the effects produced. Saussure, jun., supposes 

 that the tubes contract as soon as they have received the sap 

 in the root, and that this contraction, commencing in the 

 root, proceeds upwards, impelling the sap before it : and I 

 have suggested that the expansion and contraction of the 

 compressed cellular, or laminated substance (the tissu cel- 



* Philosophical Transactions for 1801, p. 330'. 



lulaire 



