ee 
ADHESIVE DISCS OF ERCILLA VOLUBILIS. 99 
the vascular tissue of the disc gradually merge into those forming 
the cortex of the stem (fig. 6). The fact that the discs remain 
functional after the formation of a complete periderm seems to 
show that it is not merely the casting off of the disc, as is the 
case in those tendrils of Ampelopsis which do not come into 
contact with a support. 
The Formation of Roots on the Disc.—Some dises growing in a 
moist atmosphere, after their periderm is completely formed, 
give rise to small roots. These arise from the cells below the 
cork cambium, and force themselves out through the corky 
tissue above (fig. 7). In structure they resemble normal roots, 
having a root-cap and a well-developed large central cylinder 
which is in connexion with the vascular tissue of the disc, and 
thus indirectly with that of the stem. They entirely differ from 
the normal root, however, in that the outer layers of the root- 
cap and the whole of the cortical cells have thick suberized 
walls. This suberization takes place very early, so that the root 
1s always completely surrounded by several layers of corky cells. 
The majority of the discs do not bear roots at all, while in some 
cases as many as six project from one disc. They grow to little 
more than half an inch and then remain stationary. There is 
no sign of these roots assisting in any way towards the support 
of the stem, and it is difficult to imagine what function they 
could possibly perform. 
In time, as the stem becomes thicker and heavier, the disc 
ceases to function, and by the friction of the branch against the 
“upport, the remains of the hairs and most of the original outer 
cols are worn away, down to the periderm formed by the cork 
die nd Thus, on an old part of the stem, the position of the 
the MN icated by a brown scar, which projects very little from 
ace of the stem. 
fusci mentary and Abortive Discs.—In addition to the normal 
the plant "i there are many others, of no apparent service to 
The condit various stages of development and degeneration. 
discs var m governing the formation of these rudimentary 
necessary for mur ready suggested, absolute contact seems to be 
stimulates», u „ evelopment, although proximity to the support 
on branches shi to a large extent. On the other hand, discs 
tionately en, ich grow away from the support become propor- 
leave Y smaller the farther they are removed from it. Since the 
šare arranged in a 2 spiral, and the internodes are compara- 
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