190 THEORY OF VEGETATION 



a great many kinds of plants, the cotyledons out of the soil, which then 

 become the seminal leaves of the young plant*. During this period the 

 young plant derives nutriment almost wholly from the cotyledons or seed- 

 leaves, and if those be destroyed it perishes. Gravitation, by operating 

 on bodies differently organised 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 f. 



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 

 the 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 circle, upon which the alburnum is deposited by the bark, in the form 

 of wedges, or like the stones of an arch j. 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 . 



A considerable part of the ascending fluid must necessarily 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 to secrete into the alburnum through passages 

 correspondent to the anastomosing vessels of the animal economy ||. For 

 as the cotyledons or seed-leaves first afforded the organisable matter 

 which composed the first proper leaves, so these, when full-grown, prepare 

 the fluid which generates other young leaves ; the health and growth of 

 which are as much dependent on the older leaves as those, when first 

 formed, were upon the cotyledons ^|. 



The power of each proper leaf to generate sap, in any given species 

 and variety of plant, appears to be in the compound ratio of its width, 



* See above, No. VII. t Above, No. VII. J Above, No. II. Above, Nos. II. and V. 

 || Above, No. IX. fl Above, No. V. 



