THEORY OF VEGETATION. 



351 



respond with those of the bark of the future tree, and arc 

 indeed perfect cortical vessels *. From the point of the First root, 

 caudex springs the first root, which, at this period, consists 

 wholly of bark and medulla, without any alburnous or 

 woody matter; and, if uninterrupted by any opposing 

 body, it descends in a straight line towards the centre of 

 the Earth, in whatever position the seed has been placed, 

 provided it has been permitted to vegetate at rest +. 



Soon after the first root has been emitted, the caudex Lengthening of 

 elongates, and taking a direction diametrically opposite to thecaudex - 

 that of the root, it raises, in a great many kinds of plants, 

 the cotyledons out of the soil, which then become the se- 

 minal leaves of the young plant + . During this period the 

 young plant derives nutriment almost wholly from the co- 

 tyledons or seed-leaves, and if those be destroyed, it perishes. 

 Gravitation by operating on bodies differently organized, 

 and of different modes of growth, appears at once the cause 

 why, in the preceding case, the root descends, and why 

 the elongated plumule ascends §. 



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

 depositing alburnous or woody matter; and as soon as this root - 

 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 New set of res- 

 unfold, and a set of vessels, which did not exist in the root, ses " 

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

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

 and the bark, form a circle, upon which the alburnum is 

 deposited, by the bark, in the form of wedges, or like the 

 stones of an arch ||. 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, be- 

 comes converted into the true sapor blood of the plant: True sap, 

 and as this fluid, during germination, descended from the 



* Ibid. 1809: Journ. vol. XXV, p. 18. 



f Phil. Trans. 1809, 1st part, p. 170: Journ. vol. XXV, p. 119. 



X Phil. Trans. 1806. 



§ Phil. Trans. 1st part, 1806, p. 4: Journ. vol. XIV, p. 409". 



jj Phii Trans. 1801, plate 27th. 



. cotyledons 



