ON THE ACTION OF ALBURNOUS VESSELS. 131 



by supposing that a small part of the true sap, descending from the 

 leaves, escapes downwards through the porous substance of the albur- 

 num. Several facts stated by Hales, seem favourable to this supposition; 

 and the existence of a power in the alburnum to carry the sap in differ- 

 ent directions, is proved in the growth of inverted cuttings of different 

 species of trees*. But I have derived so many advantages, both as 

 gardener and farmer, (particularly in the management of fruit and forest 

 trees,) from the experiments which have been the subject of my former 

 memoirs, that I am confident much public benefit might be derived 

 from an intimate acquaintance with the use and office of the various 

 organs of plants ; and thence feel anxious to adduce facts to prove 

 that the conclusions I have drawn, are not inconsistent with the facts 

 stated by my great predecessors. 



It has been acknowledged, I believe, by every naturalist who has 

 written on the subject, (and the fact is indeed too obvious to be contro- 

 verted,) that the matter which enters into the composition of the radicles 

 of germinating seeds existed previously in their cotyledons, and as the 

 radicles increase only in length by parts successively added to their 

 apices, or points most distant from their cotyledons, it follows of necessity, 

 that the first motion of the true sap, at this period, is downwards. And 

 as no alburnous tubes exist in the radicles of germinating seeds during 

 the earlier periods of their growth, the sap in its descent, must either 

 pass through the bark, or the medulla. But the medulla does not 

 apparently contain any vessels calculated to carry the descending sap ; 

 while the cortical vessels are during this period much distended and full 

 of moisture ; and as the medulla certainly does not carry any fluid in 

 stems or branches of more than one year old, it can scarcely be suspected 

 that it, at any period, conveys the whole current of the descending sap. 



As the leaves grow, and enter on their office, cortical vessels, in every 

 respect apparently similar to those which descended from the cotyledons, 

 are found to descend from the bases of the leaves ; and there appears 

 no reason with which I am acquainted, to suspect that both do not carry 

 a similar fluid, and that the course of this fluid is, in the first instance, 

 always towards the roots. 



The ascending sap, on the contrary, rises wholly through the alburnum 

 and central vessels ; for the destruction of a portion of the bark, in a 

 circle round the tree, does not immediately, in the slightest degree check 

 the growth of its leaves and branches ; but the alburnous vessels appear, 

 from the experiments I have stated in a former paperf, and from those 



* See above, No. IV. f See above, No. IV. 



K2 



