in the Genus Primula. 3 1 7 
deavoured to attain a similar end. We may therefore suppose 
that, as a first resort, the original vascular bundle was enlarged 
to the greatest size that such a collateral structure could 
possibly attain compatible, at the same time, with economy 
of space. This point reached, it became subdivided into 
several strands. Then, in P. japonica and P. denticulata , 
a still further increase in the amount of vascular tissue present 
was obtained by the production of bundles on the inner side 
of certain of the traces, thus converting them into perfect 
steles. 
Transitional Phenomena in the Seedling. 
The epicotyledonary region of the stem in these plants is 
monostelic, with a narrow pithless central cylinder, in which 
no separate vascular bundles can be clearly distinguished. 
The structure remains the same also for some little distance 
up the stem, except that the central cylinder increases slightly 
in diameter owing to the appearance of a few pith-cells at 
its centre. Two different types of development are to be 
found in the seedlings of P. japonica which are parallel to 
the two types of structure present in the mature stem. 
Seedlings exhibiting the first type would no doubt grow into 
those mature plants in which the imperfect form of stele 
is at least greatly predominant, if not exclusively present. 
In these the central cylinder of the epicotyl is retained in 
a perfectly normal condition until the departure of the seventh 
or eighth leaf-trace at least ; in fact, so far as the differentia- 
tion of the tissues renders any decision possible. The first 
three or four traces in passing out leave a small gap of two 
or three cells only ; and although the higher traces may leave 
a somewhat wider gap, still the endodermis invariably passes 
directly across the opening, and does not become folded into 
the interior of the cylinder (Fig. 7). Thus the gap does not 
really affect the stele as a whole, but only the ring of vascular 
bundles. In fact, it is only an unusually large medullary ray 
which becomes filled up again by the more or less rapid 
