310 G wynne- Vaughan . — On Polystely 
of internal bundles. The internal endodermis is readily 
observed in P. involucrata , though not (for reasons mentioned 
above) in P. obtusifolia. 
Imperfect steles of a slightly different kind are also to be 
found in P. involucrata and P. denticulata , but, although they 
are constantly to be met with in all regions of the stem, the 
result is not so far-reaching and disturbing in its effect upon 
the general anatomy of the plant as in the case just described 
for P. japonic a. The chief point of difference between the 
imperfect steles found in these two plants and that in P.japonica 
is that here the internal vascular bundles are absent only 
from the median portions of the stele, while they are present 
for some little distance around each corner. In the median 
region, where the internal bundles are wanting, the internal 
endodermis drops in towards the xylem of the external 
bundles, until it is separated from their protoxylems by one 
or two layers of pericycle only (Fig. 4). 
The existence of this kind of imperfect stele is explained 
by an examination into the manner in which the leaf-traces 
take up their place in the ring or circle of vascular tissue in 
the stem. They do so at the lower angle of each mesh 
in the network of steles, coming in between two perfect steles 
as they approach each other towards the lower end of the mesh 
(Fig. 2). There the leaf-trace fuses laterally, at the same 
time, with both of these steles (Fig. 3) so as to form a large 
arc, in which, of necessity, the internal vascular bundles which 
should be present just within the leaf- trace are wanting, while 
those on the inner side of the two perfect steles which are 
connected by the leaf-trace are, of course, present (Fig. 4). 
However, as this imperfect stele passes down the stem, 
internal bundles appear in the median region also, on the 
inner side of those bundles which came in from the leaf-trace, 
and thus the large arc becomes a perfect stele. Lower down 
still it divides by constriction into two smaller, but at the 
same time perfect, steles, which are now ready to undergo 
a similar process again with a lower leaf-trace (cf. the steles 
in Fig. 2). 
