353 THEORY OP VEGETATION, 



cotyledons and seed-leaves of the plant, it now descends 

 from its proper leaves, and adds, in its ib- it, to the bulk 

 Alburnum in of the stem, and the growth of the roots. Alburnum is 

 other^cenTl a ' SO deposited ' n tne stem °f ^e Pl aQ tj below the proper 

 vessels. leaves, as it was previously deposited below the seed-leaves, 



and from this spring other central vessels, which give ex- 

 istence to, and feed other leaves and buds *. 

 Fluid absorbed A considerable part of the ascending fluid must neces. 



from the soil sar iu- have been recently absorbed from the soil: but in the 



mixed with sap J J 



inthealbur- alburnum it becomes mixed with the true sap of the plant, 



aum - a portion of which, during its descent down the bark, ap- 



pears to secrete into the alburnum, through passages cor- 

 respondent to the anastomosing vessels of the animal eco- 

 nomy +. For as the cotyledons, or seed-leaves, first af- 

 forded the organizable matter which composed the first pro- 

 per 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 J. 

 Power of the The power of each proper leaf to generate sap, in any 

 leaves to gene- gj ven S p ec j es an d variety of plant, appears to be in the com- 

 pound ratio of its width, its thickness, and the exposure of 

 its upper surface to light, in proper temperature. As the 

 growth of the plant proceeds, the number and width of the 

 inaturelcaves increase rapidly, in proportion to the num- 

 ber of young leaves to be formed; and the creation con- 

 sequently exceeds the expenditure of true sap. This there- 

 fore accumulates during a succession of weeks, or months, 

 or years, according to the natural habits and duration of 

 the plant, and varying considerably according to the soil 

 and climate in which each individual grows: and the sap 

 thus generated is deposited in the bulb of the tulip, in the 

 tuber of the potato, in the fibrous roots of grasses, and in 

 the alburnum of trees, during winter, and is dispersed 

 through their foliage and bark during the spring and sum» 

 mer§. 



* Phil. Trans. 1801 and 1805. 



t Phil. Trans. 1807, p. 109 : Journ. vol. XIX, p. 24G^ 



% Ibid. 1805: Journ. vol. XII, p. 233. 



§ Ibid. 1809, p. 10: Journ. vol. XXV, p. 123. 



As 



