432 Boodle .- — The Mode of Origin and the Vascular Supply of 
The tissue produced by division of the sub-epidermal cells gives rise 
to the internal tissues of the leaf-rudiment, the epidermis of the latter being 
derived from that of the tuber. The portion of epidermis outside the mass 
of small cells belonging to the early stage of the leaf-rudiment (see Fig. i), 
no doubt, stretches slightly at first, and then, as growth beneath it continues,, 
it must undergo rejuvenescence, its cells (or a central group of them) growing 
and dividing so as to keep pace with the extension of the internal tissues. 
The original cuticle belonging to this epidermis apparently becomes ex- 
foliated during the enlargement of the rudiment, pieces of partially detached 
cuticle having been observed in some cases. 
The above description refers to specimens in which no periderm had 
been produced. Some of the tubers examined, however, were older and 
had begun to form periderm apparently before the origin of some of the 
adventitious leaves. One or two early stages of leaf-rudiments were 
examined, and found to lie immediately beneath the layer of cork-cells, 
and in lateral contact with the unsuberized layers of the periderm. Later 
stages showed the cork-layer ruptured, and the young leaf .protruding. As 
the periderm arises in the sub-epidermal layer of the tuber, the epidermis 
of the leaf, in the cases under consideration, is consequently not derived 
from that of the tuber, but, together with the inner tissues of the leaf, from 
portions of the cells of the sub-epidermal layer, or from products of the 
phellogen. 
Thus, taking the cases of the young and older specimens together, it 
appears that the internal tissues of the leaf are derived from the sub- 
epidermal layer of the tuber, either directly or indirectly, but that the 
epidermis of the leaf may have either an epidermal or sub-epidermal 
origin. 
At the stage of the leaf-rudiment shown in Fig. i, no procambial 
strand has been formed, but the appearance of a procambial connexion 
with the stele of the tuber is not long delayed. In Fig. a a rudiment of 
a leaf about 0*5 mm. in length is seen to be connected with the central 
cylinder by a procambial leaf-trace. The formation of the procambial 
tissue progresses from without inwards, i. e. from the leaf-rudiment towards 
the stele, and the progress is presumably rapid, only a few examples of 
traces stopping short of the vascular ring having been observed. One 
of the traces referred to reached the endodermis, and one died out half-way 
across the cortex. 
The cells of the procambial trace are formed by repeated divisions in 
the rather large parenchymatous cells of the cortex of the tuber. A trans- 
versely cut procambial trace (perhaps not yet complete), shown in Fig. 3, 
is seen to have arisen by the subdivision of a single cell, at the level of the 
section figured. A portion of another trace cut longitudinally is represented 
in Fig. 4, in which the appearance is suggestive of ordinary cambial divisions. 
