West. — A Contribution to the Study of the Marattiaceae. 371 
as well as with one another. On the other hand, in accordance with its 
dorsiventral organization, only a single weakly developed leaf-trace (i. e. l.t. 8 
in PL XXI, Fig. 1 A, and PL XXII, Fig. 7) takes its origin from the 
ventral surface of the stem-stele, although a number of large diamond- 
shaped gaps, closely resembling leaf-gaps, occur in this part of the stele. 
In Danaea the internal vascular cylinder arises by the gradual 
elaboration of the original commissural strand. In its development the 
internal vascular system passes through a series of stages strictly analogous 
to those of the original vascular cylinder. For comparison, Text-figs. 4, A 
and 4, B, which represent respectively a model of the stelar system of the 
rhizome of a young sporophyte of Danaea alata and a model of the internal 
stelar system of the rhizome of an adult plant of the same species, are 
placed side by side. 
It has already been stated that the commissural strand (Fig. 1, A, c.s .) 
at first consists of a small solid vascular bundle which pursues a somewhat 
zigzag course through the medullary ground-tissue, fusing with the main stelar 
system at the apex of the leaf-gaps, which it helps to close ; in other words, 
this commissural strand itself functions as a compensating [Ersatz) strand. 
Traced upwards, this strand rapidly gains in importance, and instead of 
fusing directly with the outer stelar cylinder at the apex of the leaf-gap 
(i. e. the gap made by the departure of l.t. 5 in PL XXI, Fig. 1 A), it gives 
off a branch (Text-fig. 4, B, comp.s. 1) which functions as the compensating 
strand. The commissural strand then crosses over towards the upper end 
of the next leaf-gap above ; meanwhile, the number of its vascular elements 
are considerably augmented by the addition of those of a root-trace (Text- 
fig. 4, B, r.t. 2) which passes through this foliar gap and fuses with the 
commissural strand. In this way a large solid mass of vascular tissue, 
more or less oval in transverse section, is produced. A short massive 
compensating strand (Text-fig. 4, B, comp.s . 2) leaves this vascular mass 
and, anastomosing right and left with bundles of the external cylinder, 
assists in closing the gap formed by the exit of the meshed segment, which 
at this stage constitutes the leaf-trace. The departure of this compensating 
strand produces a distinct gap, comparable with a leaf-gap, in the central 
stelar system, which now opens out to form an incomplete cylinder of 
vascular tissue. Text-fig. 6, i— v, represents a successive, but not consecutive, 
series of transverse sections of this transitional region (indicated by a x 
in PL XXI, Fig. 1 a) ; it is seen that a well-marked ‘ pocket ’ is produced, 
at first phloem (Text-fig. 6, i) and then characteristic ground-tissue 
parenchyma (Text-fig. 6 , ii, iii) appearing in the centre of the xylem core. 
The subsequent stages in the development of the internal stelar 
system proceed along lines essentially similar to those of the original 
solenostele ; in brief, the internal vascular cylinder, thus inaugurated, 
increases in siz z pari passu with the widening of the outer cylinder and of 
