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XV, On Circulation and the Formation of Wood in Plants. By HERBERT SPENCER, Esq. 
Communicated by GEORGE Busk, Esq., F.R.S., Sec. L.S. 
(Plate LIV.) 
Read March 1st, 1866. 
OPINIONS respecting the functions of the vascular tissues in plants appear to 
make but little progress towards agreement. The supposition that these vessels and 
strings of partially-united cells, lined with spiral, annular, reticulated, or other frame- 
Works, are carriers of the plant-juices, is objected to on the ground that they often 
contain air: as the presence of air arrests the movement of blood through arteries 
and veins, its presence in the ducts of stems and petioles is assumed to unfit them as 
channels for sap. On the other hand, that these structures have a respiratory office, as 
some have thought, is certainly not more tenable, since, if the presence of air in them 
negatives the belief that their function is to distribute liquid, the presence of liquid in 
them equally negatives the belief that their function is to distribute air. Nor can any 
better defence be made for the hypothesis which I find propounded, that these parts 
serve “to give strength to the parenchyma.” Tubes with fenestrated and reticulated 
internal skeletons have, indeed, some power of supporting the tissue through which they 
pass; but tubes lined with spiral threads can yield extremely little support, while tubes 
lined with annuli, or spirals alternating with annuli, can yield no support whatever. 
Though all these types of internal framework are more or less efficient for preventing 
closure by lateral pressure, they are some of them quite useless for holding up the mass 
through which the vessels pass ; and the best of them are for this purpose mechanically 
inferior to the simple eylinder. The same quantity of matter made into a continuous 
tube would be more effective in giving stiffness to the cellular tissue around it. id 
In the absence of any feasible alternative, the hypothesis that these | vessels are distri- 
butors of sap claims reconsideration. The objections are not, I think, so serious as they 
seem. The habitual presence of air in the ducts that traverse wood, can scarcely be 
held anomalous if when the wood is formed their function ceases. The canals which 
tamify through a Stag’s horn, contain air after the Stag’s horn is fully developed; but 
lt is not thereby rendered doubtful whether it is the function of arteries to convey blood. 
Again, that air should frequently be found even in the vessels of petioles and leaves, will 
hot appear remarkable when we call to mind the conditions to which a leaf is subject. 
Evaporation is going on from it. The thinner liquids pass by osmose out of the vessels 
Into the tissues containing the liquids thickened by evaporation. And as the vessels 
are thus continually drained, a draught is made upon the liquid contained in the stem 
and roots, Suppose that this draught is unusually great, or suppose that around the 
VOL. xxy, 3 L 
