372 Chandler . — On the Arrangement of the Vascular Strands 
parenchyma is differentiated between them, the endodermis being pushed, 
as it were, before the advancing ground-tissue (Fig. 25). The two vascular 
strands (the previous leaf-gap has not yet been closed) do not at this level 
become completely separated by the ground-tissue, so that we have, in 
transverse section, two patches of xylem each surrounded by phloem, the 
whole being ensheathed in a common pericycle and endodermis (Fig. 25). 
At the moment when the complete separation of the two vascular 
strands by the ground-tissue seems almost effected, a concentric strand is 
gradually nipped off from each of the horns bounding the enlarged sixth 
break in the vascular tissue ; sometimes the strands were found to be nipped 
off simultaneously, but in other plants they became free from the cauline 
system at different levels, a state of affairs found to be very common in 
connexion with such multiple leaf-traces. 
The two strands, each of which is surrounded by its own endodermis, 
never fuse together, as in previous cases, but remain quite separate and pass 
out to form the first double leaf-trace. During the complete separation of 
this trace, the larger of the two cauline strands begins to divide into two 
(Fig. 28), one portion fusing with the remaining main strand and thus 
closing, at last, the fifth leaf-gap. Fig. 29 shows that at this level the 
cauline system consists of two concentric vascular strands each surrounded 
by a separate endodermis. Subsequently a double leaf-trace is formed in 
the manner indicated in Figs. 29-31, and we have the final appearance of 
three cauline strands by the division of one of the original two, the actual 
changes being represented in Figs. 33-35. 
Subsequent developments consist of a gradual elaboration of the 
vascular system, but the processes involved in the actual formation of 
the leaf-traces are essentially the same as those described above ; that is to 
say, we have a splitting of the vascular tissue followed by the separation 
of the two petiolar strands from the adjacent horns, and the subsequent 
closing of the previous leaf-gap. The siphonostelic character of the vascular 
system of this plant is therefore quite evident. 
A very considerable amount of variation was met with in the earliest 
transitional stages, necessitating the examination of a comparatively large 
number of specimens before the varying appearances presented could be 
reduced to some common plan. As mentioned above, the xylem of 
Lomaria gibba possesses a considerable amount of parenchyma among the 
tracheides, and it seems almost certain that the variations met with can be 
attributed to this fact, for the parenchyma cells appear in the xylem of 
the very youngest plants. The following exceptional case is interesting. 
A single parenchyma cell, quickly followed by three or four others, appeared 
in the normal diarch plate of the root. At a higher level the ordinary 
protostelic structure occurred, but the parenchyma cells now appeared as 
a band stretching across the xylem, and continuous at either end with 
