FORMATION OF WOODY FIBRE. 325 



layers of cells with a delicate spiral fibrous deposit on 

 their walls. This layer forms a kind of coat over the real 

 epidermis of the root, and is known by the name of the 

 Velamen radicum. The young shoots of Dicotyledonous 

 trees and shrubs are clothed with epidermis-like herbaceous 

 plants ; but, before the close of the past season of growth, 

 in most cases the green colour gives place to brown, which 

 is owing to the formation of a layer of cork from the outer 

 layers of cortical parenchyma. Cork is composed of 

 tubular thin-walled cells containing only air ; and some- 

 times these intercellular passages occupy a considerable 

 space, and communicate in all directions, forming a system 

 of air-spaces in the tissue. In addition there is the secre- 

 tory system, consisting of glands, simple and compound,, 

 milk-vessels, and canals filled with resins, oils, &c. 



Much of the physiological history of plant life has yet 

 to be made out : the mode in which the circulation and 

 the formation of wood are carried on is by no means a 

 settled question. Mr. Herbert Spencer observes i 1 " That 

 the supposition that certain 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 pressure of air arrests the movements of blood 

 through arteries and veins, its presence on the ducts of 

 stems and patioles 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 the presence of air in them 

 negatives the belief that their function is to distribute- 

 liquid. The presence of liquid in them equally nega- 

 tives the belief that their function is to distribute air. 

 !N"or can any better defence be made for the hypothesis 

 which I find propounded, that these parts serve 'to 

 give strength to the parenchyma.' In the absence of any 

 feasible alternative, the hypothesis that these vessels are 

 distributors of sap claims reconsideration." To obtain data 

 for an opinion on this vexed question, Mr. Spencer insti- 

 tuted a series of experiments on the absorption of dyes by 

 plants. His first experiments were failures, and it was only 



(1) Linnean Soc. Trans, vol. xxv. paf 405. 



