416 MR. SPENCER ON CIRCULATION AND 
sheath, goes on until it meets the thickenings that spread from the other bundles; and 
there is so formed an irregular eylinder of hardened tissue, surrounding the medulla and 
the vascular bundles of its sheath. As soon as this happens, these vascular bundles 
become, to a considerable extent, shielded from the effects of transverse strains, since 
the tensions and compressions chiefly fall on the developing wood outside of them. 
Clearly, too, the greatest stress must be felt by the outer layer of the developing wood: 
being further removed from the neutral axis, it must be subject to severer strains at each 
bend; and lying between the bark and the layer of wood first formed, it must be most 
exposed to lateral compressions. Among the elongated cells of this outer layer, some 
unite to form the pitted ducts. Being, as we see, better cireumstanced mechanically, 
they become greater carriers of sap than the original vessels, and, in consequence of 
this, as well as in consequence of their relative proximity, become the sources of nutri- 
tion to the still more external layers of wood-cells. The same causes and the same 
effects hold with each new indurated coat deposited round the previously indurated coats. 
This description may be thought to go far towards justifying the current views re- 
specting the course taken by the sap. But the justification is more apparent than 
real. In the first place, the implication here is that the sap-carrying function is at 
first discharged entirely by the vessels of the medullary sheath, and that they cease 
to discharge this function only as fast as they are relatively incapacitated by their 
mechanical circumstances. And the second implication is, that it is not the wood 
itself, but the more or less continuous canals formed in it which are the subsequent 
sap-distributors. This, though readily made clear by microscopic examination of the 
large pitted ducts in a partially lignified shoot that has absorbed the dye, is less 
manifestly true of the peripheral layer of sap-carrying tissue finally formed. But it 
is really true here. For this layer, though nominally a layer of wood, is practically 8 
layer of inosculating vessels. It is formed out of irregular lines and networks of 
elongated pitted cells, obliquely united by their ends. Examination of them after ab- 
sorption of a dye, shows that it is only along the continuous 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 sj* 
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 2 
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 throug! 
the new tissue are composed of cells that pass through modified forms into the sp! 
vessels of the new medullary sheath. This transition may be still more clearly t^ 
in a terminal bud that continues the line of last year’s shoot. A longitudinal section " 
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 i 
inactive), but that their supplies are obtained from those inosculating canals formed ] 
eM nies layer of prosenchyma, and that between the component 
e new vascular system there are all gradations of structure ". 
* It eae i p^ B 
may be added here that, on considering the mechanical actions that must go on, we are enabled in som * i 
