CIRCULATION AND FORMATION OF WOOD IN PLANTS. 581 



vessels. It is formed out of irregular Hues 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 

 must go on, we are enabled in some measure 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- 

 chyma-cells 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 producing 

 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 inter- 

 pretation, 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 



