20 
Lang—Studies in the Morphology and 
all round the stele, although the branch stele is not yet reached, while in the 
last section (f) it is still present all round the stele, although the branch has 
passed off and the next leaf-trace has reached the surface of the rhizome. 
Its appearance in this region beyond the departure of the branch is shown 
in PL II, Photo 25, which is to the same scale, and may be compared with 
the ordinary primary structure seen in PL I, Photo 4. A portion of the 
stelar tube, showing the accessory xylem, is more highly magnified in 
Photo 26 on PL II. It will be sufficient to refer to this last figure, which 
should be compared with PL I, Photo 1, in the following description. 
The endodermal markings were not pronounced in the necrosed tissues 
of this rhizome, but the approximate position of the endodermis is indicated 
at e . Within this comes the pericycle, and at ph. the phloem is clearly 
evident. Within this comes conjunctive parenchyma. In an ordinary 
rhizome the outer metaxylem (. x.o .), would have come next followed by 
the inner metaxylem (x.i.) and the pith. The position of these tissues is 
indicated in PL II, Photo 26, but to the outside of the outer metaxylem 
the irregular band of accessory xylem (x 2 ) is present in addition. This has 
evidently been secondarily developed as the result of tangential divisions 
taking place in the cells of the conjunctive parenchyma adjoining the 
primary xylem, and cells showing tangential division-walls are evident in 
the photograph. Many of the resulting elements have developed into 
tracheides, but others have remained parenchymatous. It is due to this 
that the accessory xylem is at places continuous with the primary xylem, 
while at other places some parenchymatous secondary tissue intervenes (cf. 
Photo 25, PL II). This xylem was clearly a secondary development after 
the primary structure of the stele was completed. It occupies the usual 
position of a normal zone of secondary xylem relatively to the primary 
xylem and phloem. On the other hand it is irregular, in that no definite 
meristematic layer or cambium is formed or persists outside the newly 
formed xylem. Even allowing for these peculiarities, it seems to me clear 
that the development of this accessory xylem should rightly come under 
the conception of secondary thickening. It is of special interest that in 
Helminthostachys , where no secondary thickening is normally found, we 
can correlate the development of this accessory xylem with the influence 
exerted by the development of a branch from a vestigial bud upon the 
completed tissues of the main stele. 
Turning now to consider the relations of the branch to the main 
rhizome, reference may first be made to Text-fig. 5. This represents in 
outline a transverse section of the main rhizome, showing the branch 
growing so nearly at right angles to it that its stele is cut longitudinally 
throughout its course. The xylem is represented in black without any 
distinction of inner, outer, or accessory xylem. The leaf-trace after the 
one which subtended the branch is seen nearing the periphery of the 
