By Thomas Andrew Knight, Esq. 219 



preceding case, the root descends, and why the elongated 

 plumule ascends.* 



The bark of the root now begins to execute its office of 

 depositing alburnous or woody matter ; and as soon as this is 

 formed, the sap, which had hitherto descended only through 

 the cortical vessels, begins to ascend, through the alburnum. 

 The plumule in consequence elongates, its leaves enlarge and 

 unfold, and a set of vessels, which did not exist in tiie root, 

 are now brought into action. These, which I have called the 

 central vessels, surround the medulla, and, between it and the 

 bark, form a eircle upon which the alburnum is deposited, 

 by the bark, in the form of wedges, or like the stones of an 

 arch.-f* Through these vessels, which diverge into the leaf 

 stalks, the sap ascends, and is dispersed through the vessels, 

 and parenchymatous substance of the leaf; and in this organ 

 the fluid recently absorbed from the soil, becomes converted 

 into the true sap or blood of the plant : and as this fluid, 

 during germination, descended from the cotyledons and seed- 

 leaves of the plant, it now descends from its proper leaves, 

 and adds, in its descent, to the bulk of the stem, and the 

 growth of the roots. Alburnum is also deposited in the stem 

 of the plant, below the proper leaves, as it was previously 

 deposited below the seed- leaves, and from this spring other 

 central vessels, which give existence to, and feed other leaves 

 and buds.J 



A considerable part of the ascending fluid must neces- 

 sarily have been recently absorbed from the soil : but in the 

 alburnum it becomes mixed with the true sap of the plant, a 

 portion of which, during its descent down the bark, appears 



• Philosophical Transactions, 1806. f Ibid, 1801. % Ibid. 1801 and 180a. 



