APPENDIX a 



[From the Transactions of the Linnean Society, vol. xxv.] 



XV. On Circulation and the Formation of Wood in Plants. By 

 Herbert Spencer, Esq. Communicated by George Busk, 

 Esq., F.B.S., Sec. L.S. 



Read March 1st, 1866. 



Opinions respecting the functions of the vascular tissues in plants 

 appear to make but little progress towards agreement. The suppo- 

 sition that these vessels and strings of partially-united cells, lined 

 with spiral, annular, reticulated, or other frameworks, 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 dis- 

 tribute 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 cylinder. 

 The same quantity of matter made into a continuous tube would be 

 more effective in giving stiffness to the cellular tissue around it. 



In the absence of any feasible alternative, the hypothesis that 

 these vessels are distributors of sap claims reconsideration. The 

 objections are not, I think, so serious as they seem. The habitual 



