1898-99.] Edith Chick on Kicinus Communis. 
663 
next above these were developed at a later date in a ring cor- 
responding to the second node, when in turn this was just below 
the growing point, and so on. These, on their downward course, 
anastomosed with the already existing bundles a, b, c, d , e, /, at 
the level X Y (fig. 8). 
The External Conjunctive Tissue. 
In 1893, Flot* made an important contribution to the delimita- 
tion of the regions of the stele typical of dicotyledonous and coni- 
ferous stems. He distinguished an £ internal ’ from an £ external ’ 
conjunctive tissue, the former name being applied to the larger 
celled parenchyma commonly present in the centre of the stele, 
the latter to the tissue immediately investing the vascular ring. 
The external conjunctive may be divided into (1) pericycle, the 
peripheral layer of the stele ; (2) rays, the tissue between the 
bundles ; and (3) perimedullary zone, the layer bordering the pith. 
The internal conjunctive is equivalent to pith, if the latter term is 
taken to represent a naturally distinct tissue system, and is not a 
mere topographical designation for the tissue included within an 
imaginary circle, whose circumference touches the internal points 
of the primary xylems. Flot’s scheme corresponds perfectly to 
the facts in those cases where the primary bundles of the cylinder 
are separated from one another laterally by narrow bands of tissue 
(rays). He rightly insists that one cannot draw a sharp line 
between perimedullary zone and ray, or between ray and pericycle, 
when the whole of the external conjunctive is parenchymatous. 
The three zones are in that case simply topographical names for 
parts (whose limits are marked out by the bundles themselves) of 
a single tissue system investing the bundles. Histological dis- 
tinctions between them, however, frequently exist. The commonest 
of these is the partial or complete conversion of the pericycle into a 
sclerenchymatous band — a conversion often shared by the peri- 
medullary zone. 
If, now, we follow out the arrangement and structure of the 
external conjunctive in the root, transition region, hypocotyl and 
* Flot, “ Recherches sur la zone perimedullaire,” Ann des Sci. Nat. Bot., 
ser. 7, 1893. 
