ON THE STEM OF DAMMARA ROBUSTA. 445 
of the articulate stems of Crassula arborescens, the separation of 
parts is at times followed by root-formation from the pieces so 
shed and their subsequent growth; and it is possible, even, that 
the ordinary fall of leaves has a similar evolutionary history. ' 
In Cotyledon Hookerii I have often seen the fallen leaves serve 
a8 adventitious organs of propagation; and the same is true of 
Bryophyllum, whilst the striking of detached Orange and Begonia 
leaves is a well-known phenomenon. Mr. Thiselton Dyer tells 
me that the branches torn off by the wind or by accident from 
the arboreal Aloe of Caffraria (A. Barbera) are said to take root. 
The branch-scar, when examined immediately after disarticu- 
lation, is ovoidal, concave, and has a finely granular surface; the 
narrow circular zone of the fractured wood projects slightly at 
the bottom of the cicatricial fossa, and in the cortical parenchyma 
are embedded the ruptured ends of the bast-fibres. 
In the subsequent history of the scar, as traceable by dissection*, 
the fractured surface of the wood becomes covered over and 
Closed in by a thin extension of the surrounding parenchyma, a 
low oval or horseshoe-shaped ridge remaining on the cicatricial 
surface to mark the situation of the closely subjacent fractured 
wood. At the same time the surface of the scar becomes smooth 
and shining from the further growth of its investing corky layer; 
and tbis may, in process of time, fissure like that which covers 
the general surface. The remains of the wood and bast-fibres 
passing through the parenchymatous system of the scar perma- 
nently retain their position in and connection with the superficial 
Parts of the scar. 
No further growth, however, of this wood and bast in the mid- 
substance of the ground-tissue of the scar occurs; and the addi- 
tion of wood to the general surface of the stem has the remark- 
able effect of rupturing or dissociating this wood in the scar 
from the wood of the stem with which it was originally related. 
In the history of a leaf-scar the same thing happens; that is to 
Say, on the accession of general growth, the remains of the closed 
bundles of the petiole, in what has now become, by the separation 
of the leaf, a part of the general cortex, retain their connection, 
the more superficial parts with the tissue of the scar, the deeper 
with the central wood and medullary sheath, with which they are 
M * I have presented a series of preparations illustrating this history to the 
useum of the Royal Gardens, Kew. 
