in the ‘ Seedlings' of Certain Leptosporangiate Ferns . 387 
and 178, in which it is seen that the bulged portion of the vascular tissue 
goes out as the leaf-strand, resulting in the union of the inner and outer 
endodermis. The gap remains open for some considerable distance, the 
vascular ring increasing in diameter, and, before the gap is closed, the next 
leaf-trace is given off from the back of the xylem arc in the manner 
indicated in Fig. 179, with the result that the cauline system now consists 
of two strands. The xylem of the leaf-trace is at first in two distinct 
strands, a fact hinting at the double origin of the later traces. Soon after 
the exit of the petiolar bundle, the previous leaf-gap closes as far as the 
vascular tissue as a whole is concerned, the horns of the xylem, however, 
not fusing, but remaining separated by two or three parenchyma cells 
(Fig. 180). This is no doubt to be regarded as a preparation for the next 
leaf-trace, which is definitely double, as shown in Fig. 181. The closing of 
the previous gap is immediately followed by the breaking of the cauline 
vascular strand into two (Figs. 182, 183). The following leaf-trace is double, 
the two strands being nipped off from the vascular horns resulting from the 
fractionation of the cauline system (Fig. 184). Two or three rootlets join 
the stem at this level, and one of the vascular strands then breaks into two 
portions, a division which is rapidly followed by a fusion of one portion 
with the undivided strand, and the subsequent formation of another leaf- 
trace as shown in Figs. 185-187. 
The two cauline strands then effect a junction, but quickly separate 
again. As will be seen from Fig. 187, the diameter of the vascular tube is 
only slightly less than that of the stem as a whole, the ground-tissue being 
reduced to a mere band, the outer layers of which are very strongly 
sclerosed. The xylem has increased in width, but is still comparatively 
narrow ; the phloem is normal, and the pericycle maintains its multiseriate 
character. The endodermis is extremely well marked, especially that 
portion of it lining the concavity of the vascular arc, a condition of things 
no doubt to be correlated with the fact that large numbers of intercellular 
spaces occur in the loosely arranged central ground-parenchyma. Before 
the last leaf-gap is closed, the vascular arc again breaks into two, and a new 
double leaf-trace is formed in the manner already described. The two 
bundles, which are separated by a strand of sclerenchyma, leave the cauline 
system very gradually, and their separation from it is not complete until 
the previous leaf-gap has been repaired. At a level immediately above, 
the vascular band again breaks, first into two and subsequently into three 
parts (Fig. 189), and during its division several roots join the vascular 
system. After the entry of the roots, the two largest of the vascular 
strands again unite (Fig. 190), and, while the exit of another leaf-trace 
is being effected, the remaining cauline strand also effects a junction with 
the curved band thus formed. 
Subsequent changes are of a very interesting character, and result from 
