550 



vessels. It is formed out of irregular lines and networks of elon* 

 gated pitted cells, obliquely united by their ends. Examination of 

 them after absorption of a dye, shows that it is only along the con- 

 tinuous channels they unite to form that the current has passed. 

 But the essentially vascular character of this outer and latest-formed 

 layer of the alburnum is best seen in the fact that the vascular sys- 

 tems of new axes take their rise from it, and form with it continuous 

 canals. If a shoot of last year in which growth is recommencing, be 

 cut lengthways after it has imbibed a dye, clear proof is obtained 

 that the passage of the dye into a lateral bud takes place from this 

 outermost layer of pitted cells, and that the channels taken by the 

 dye through the new tissue are composed of cells that pass through 

 modified forms into the spiral vessels of the new medullary sheath. 

 This transition may be still more clearly traced in a terminal bud 

 that continues the line of last year's shoot. A longitudinal section 

 of this shows that the vessels of the new medullary sheath do. not 

 obtain their sap from the vessels of last year's sheath (which, as 

 shown by the non-absorption of dye, have become inactive), but that 

 their supplies are obtained from those inosculating canals formed out 

 of last year's outermost layer of prosenchyma, and that between the 

 component cells of this and those of the new vascular system there 

 are all gradations of structure.* 



* It may be added here that, on considering the mechanical actions that 

 rnnstgo on, we are enabled in somemeasnre to understand both how such inos- 

 culating channels are initiated, and how the structures of their component 

 cells are explicable. "What must happen to one of these elongated prosen- 

 ehyma-celis if, in the course of its development, it is subject to intermittent 

 compressions ? Its squeezed-out liquid while partially escaping laterally, 

 will more largely escape upwards and downwards ; and while repeated 

 lateral escape will tend to form lateral channels communicating with 

 laterally-adjacent cells, repeated longitudinal escape will tend to form 

 channels communicating with longitudinally-adjacent cells — so pro- 

 ducing continuous though irregular longitudinal canals. Meanwhile 

 each cell into and out of which the nutritive liquid is from time 

 to time squeezed through small openings in its walls, cannot thicken 

 internally in an even manner : deposition will be interfered with by 

 the passage of the currents through the pores. The rush to or from each 

 pore will tend to maintain a funnel-shaped depression in the deposit around ; 

 and the opening from cell to cell will so acquire just that shape which the 

 microscope shows up— two hollow cones with their apices meeting at the 

 point where the cell -membranes are in contact. Moreover, as confirming 

 this interpretation, it may be remarked that we are thus supplied with a 

 reason for the differences of shape between these passages from one pitted 

 cell to another, and the analogous passages that exist between cells other- 

 wise formed and otherwise conditioned. In the cells of the medulla, and 

 others which are but little exposed to compression, the passages are seve- 

 rally formed more like a tube with two trumpet-mouths, one in each cell. 

 This is just the form which might be expected where the nutritive fluid 

 passes from cell to cell in moderate currents, and not by the violent_ rushes 

 caused by intermittent pressures. Of course it is not meant that in each 



