ON THE FALL OF BRANCHLETS IN THE ASPEN (POPULUS TREMULA). 809 
ruptured by the first slight violence, but the surface of the branch 
thus rudely exposed is already healed throughout by the cork, the 
ends of the vessels and the bast-fibres alone excepted—these project 
unhealed in the scar, anti eventually the periodic growth of the 
parenchyma and cork buries them beneath the surface. The pro- 
ess of healing is steirelats quite different from that by which 
a fractured surface is closed over in other exogenous trees ; in 
ese if occurs, as before noticed, os the progressive advance of 
new tissue produced from the cambiu 
Such is the structure of comparatively recent scars in the 
Aspen, bu t i in time wood is formed in the parenchyma or ground- 
tissue of = -~ cemnn the phellogen as cork, in conjunction 
with, and as an sion he — yearly addition of wood to the 
general firtste or he bra the ground-tissue solani a 
cambium, which is Seely o continued around the parenchyma 
the articular zone to the branchlet beyond, though inte cambium 
cells do not under the ordinary circumstances produce woody tissue 
n the inner side, but furnish only the elongated parenchyma of 
the articular zone aoe describe 
e reasons why the branchlets should be thus eee 
ready for such a disarticulation, and why they fall, are diffie 
render. Very often the fallen parts appear healthy, and have dias 
buds and even leaves fresh on them. Berkeley remarks, ‘* The. 
cause of death in the eanchan : is uncertain, sicnahciek 8, perhaps, 
hment which was destined for their use.’’ Th in 
the autumn; af ell-changes at the articulation have 
occur w the ground with branchlets, as i 
red a gale may 
will disarticulate leaves when ready for their fall; but the living 
process is a necessary antecedent to the detachment which does 
not own a purely mechanical causation 
Cladoptosis occurs in the other genus of the same natural order, 
viz., in the willow; and it occurs also in the oak, the scars of 
which closely ant ew those of the Aspen, and are similarly 
formed, not by an overgrowing of new tissue from the cambium, 
but by a simultaneous healing of the entire surface. Amongst 
occurs 
genera of the Artocarpacer, Antiaris towicaria (Upas tree) and 
Antiaris wate i _ as in Castilloa elastica, the india-rubber 
- tree of Central i. 
why of all $868 these few should be so constructed it is, to 
say ths least, difficult to offer epimers The natural history of 
the articulate stems of Crassula arborescens may fu rnish some clue 
demand and supply of the plant are unequal; the parent stem 
detaches soradns of its growth, which afterwards find an inde- 
pendent mode of existence. And in the well known br ophylian 
