9 
VEGETABLE PHYSIOLOGY. 
It may appear superfluous to notice a subject which has called forth the powers 
of investigation of many of the ablest philosophers of Europe ; but when we con- 
sider, that no two writers have agreed, that various and opposing functions have 
been ascribed to the same vessels, and that, to this hour, no one has been able to 
determine with accuracy, and beyond doubt, the channels through which the sap 
ascends, we may be permitted to collect and embody a few hints, which we hope 
may excite inquiry among those who have time and inclination to undertake inves- 
tigations of a nature so delicate and obscure. 
The name of that great and good man lately deceased, who during so long a 
period honourably filled the president's chair of the London Horticultural Society, 
must command attention. He it was who, perhaps, approached most nearly to 
the truth ; and we are enabled, through the communications of a friend who was 
favoured with a very extensive correspondence with Mr. Knight, to present to our 
readers some extracts copied verbatim from original letters, whence that gentle- 
man's views on many important particulars may be pretty clearly ascertained. 
The Sap Vessels — those which conduct the ascending current — have generally 
been thought to be hollow tubes : some puncturated, some spiral ; arranged in 
bundles, or more or less detached. Mr. Knight, in the earlier part of his life, 
entertained the same opinion ; and it is somewhat remarkable that the late Sir 
James Edward Smith mistook Mr. Knight's " central " vessels for those termed 
" spirals^' for he thus expresses himself: — " The same idea has been adopted, con- 
firmed by experiments, and carried to a much greater perfection by Mr, Knight, 
whose papers in the Philosophical Transactions for 1801, 1804, and 1805, throw 
the most brilliant light upon it, and I think establish no less than an entirely new 
theory of vegetation, by which the real use and functions of the principal organs 
of plants are now, for the first time, satisfactorily explained. In a young branch 
of a tree or shrub, or in the stem of an herbaceous plant, are found, ranged round 
the centre or pith, a number of longitudinal tubes, or vessels, of a much more firm 
texture than the adjacent parts, and when examined minutely, these vessels often 
appear to be constructed with a spiral coat." 
Now, Sir J. E. Smith believed the spiral vessels to be here alluded to, though 
Mr. Knight had distinctly written that, " to these " (his central) " vessels, the 
spiral tubes are everywhere appendages and again, " the spiral tubes will neither 
carry coloured infusions, nor in the smallest degree retard the withering of the 
leaf, when the central vessels are divided." 
In a letter dated September 22, 1830, Mr. Knight thus expresses his then 
opinion, and it will be pleasing to observe the honourable testimony he therein bore 
to his esteemed friend M. Dutrochet. 
" I had proved that the specific gravity of the sap of trees, in the spring^ in- 
creases in proportion to its distance from the ground, and that a good deal of 
VOL. VI. NO. LXI. c 
