Bower.—Studies in the Phytogeny of the Filie ales. 4^3 
upwards into three, peculiar to this species, takes place so that the median 
protoxylem-groups remain in the central strand of the three, while the 
lateral protoxylems continue into the two lateral strands ; in fact the abstric- 
tion is only a temporary interruption of the metaxylem, and of the tissues 
and sheaths surrounding it. In P. pycnophylla and glauca , however, there 
are usually three protoxylems in the central region of the leaf-strand as it 
passes into the basal region of the leaf, the third one (and sometimes more) 
appearing in a median position, but not by branching of the original one. 
In the lateral arms of the leaf-trace also, as it passes upwards into the leaf, 
a multiplicity of protoxylem-strands appear on the adaxial face, their 
position being indicated in the mature state by groups of cavity-parenchyma, 
as already noted. 
In following up the details of dichotomy as seen in the mature stem of 
P. semicordata , by means of serial sections successively from below up¬ 
wards, the first change is in the division of the central sclerotic core into 
two by constriction, together with the widening of the dictyostele into an 
Text-fig. 3. Five successive sections of the stock of P. semicordata, showing the disposition of 
the vascular tissues at the region of a dichotomy; the lowest section is to the left, and shows the 
central sclerotic core already divided before the stele itself becomes constricted. ( x 2§.) 
elliptical outline, but without any cessation of the usual formation of foliar 
gaps where the leaf-traces pass off (Text-fig. 3, a). When the two sclerotic 
cores have separated completely from one another, and have passed to the 
poles of the ellipse, the stele begins to encroach inwards on either side 
(Text-fig. 3, b) ; the projecting surfaces meet, and the several tissues 
successively fuse (Text-fig. 3, c ); the process is accompanied by involution 
of the outer surface of the stele (Text-fig. 3, c, d), and this is continued till 
the several tissues separate again, the division being now in a plane trans¬ 
verse to the axis of the ellipse ; the xylem separates first, then the phloem, 
and finally the sheaths. The result is the formation by fission of two 
equivalent steles (Text-fig. 3,^). Throughout the whole process leaf-gaps 
and foliar strands are formed from both of the steles, even obliquely on the 
sides of them which face one another. The structure shows plainly that it 
is a case of true dichotomy, resulting in two initially equivalent shanks. 
All the material available for the study of the anatomy of Plagiogyria 
has been of old plants, with the dictyostelic structure fully developed, but 
it has been seen above that P. pycnophylla bears stolons, arising in close 
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