808 oN THE FALL OF BRANCHLETS IN THE ASPEN (POPULUS TREMULA). 
and uniformly brown from shanget in the cork which corey it. 
The cicatrizing process is pec culiar, inasmuch as it occurs simul- 
anole! in the whole of the ieaiey but I will refer to this again. 
w what does the microscope show regarding the structure of 
the Stitenlar enlargement? How does the area through which the 
disarticulation occurs differ from the rest of the branchlet? Two 
transverse pees prepared from wood hardened in alcohol, 
aR with magenta and glycerine, washed, and mounted in 
Sef ee aye es. following differences. The section of the 
ht displays a uniformly and deeply-stained wood, traversed 
by medullary rays (of cells in single rows), and perforated with 
vessels arranged with uniform aregularity, cach slightly larger 
and in excess in the limits of the annual rings. e wood is suc- 
ceeded by a lightly-stained cambium, then by an abundance of soft 
bast, in which lie groups of bast- fibres ; through both of these the 
sraneverey elongated cells of the medullary rays are extended, 
and it the presen of these that the fine radial markings 
referred to in ee cortex of the scar and separated brengl’ss 
Contrasting with this, the section of the zone through which 
the disarticulation of the branchlet occurs presents almost solely a 
lightly-stained parenchymatous tissue, of cells square in section, 
intersected with the transversely elongated cells of medullary rays, 
and having the vessels arranged in radiating planes, instead of 
promiscuously as in the branchlet; the woody cells are very few, 
ered in groups adjoining the vessels ; in other sections the 
woody cells are eee absent; the cambium ie cortical system 
offer nothing pe 
oapindiont ed made through the enlargement show the 
wood to cease on either side of this parenchymatous zone, the 
vessels alone being continued sninterraptodly through ibs the 
same section shows the arenchyma of the articular zone to be 
composed of longitudinally elongated cells crucially iteseted by 
planes of similar cells which form the Bou Thi 
absence of parenchyma and the free presence of aaa cgi 
at once the readiness with which the veanehiatd admit of artificial 
detachment, and how it is that the construction of the parts allows 
of cell-changes by which a disarticulation may be effected similar 
to that which occurs in the active fall of a leaf. 
The changes which precede and prepare the way for the fall of 
a branchlet are essentially the same as those which precede the 
fall of the leaf or other articulate organs. A circular fissure in the 
cork along the middle of the swelling is the first outward and 
visible sign of the process proceeding wit ithin. Longitudinal sections 
made in this stage exhibit beneath the microscope a transverse 
zone of cork already formed in the p pel a dea of the swelling 
beneath the circular fissure in the 5 ede cor 
_ The bast-fibres and vessels of the wood pass through the new- 
formed cork, and are indeed the only structures which re 
connect the di part with the parent branch; both are 
