382 Chandler . — On the Arrangement of the Vascular Strands 
traces are formed in essentially the same way as those of the young plants, 
their true origin being rendered somewhat difficult of realization by the 
greatly elongated internodes. The series of changes is represented in 
Figs. 123-128. The petioles of the later leaves are generally supplied 
with two main bundles and one or two smaller ones, which, however, fuse 
into two strands soon after their entrance into the petiole. As regards 
the leaf-trace itself, then, we find that the earliest leaves are provided with 
a single strand ; later, the trace has a double origin, but a single bundle 
enters the leaf-stalk ; and, finally, the leaf-strands remain separate for 
successively longer periods until the mature condition is reached, in which 
two chief strands enter the petiole, fusing a short distance higher up. 
Several variations of the normal early transitional changes described 
above were noted. In two or three plants the xylem of the diarch plate 
broke into three parts, which were surrounded by a common phloem sheath. 
One of the xylem strands, surrounded by phloem and endodermis, passed 
out as the first leaf-trace, while the two remaining strands quickly fused, or 
separated from one another as concentric strands which reunited to form 
a single concentric rod at a slightly higher level. 
In plants of all ages it occasionally happened that small concentric 
strands left the cauline system, and, bending sharply outwards, ended 
blindly in the ground-tissue, generally just beneath the epidermis. A similar 
state of affairs was noted for Asplenium bulbiferum. 
Petiole. 
The petiole of a moderately sized leaf is almost circular in section, and 
possesses that peculiar wiriness so characteristic of many fern leaf-stalks. 
There is a single vascular strand embedded in a ground-tissue limited 
externally by an epidermis. The latter consists of a layer of cells square 
in section, and with extremely thick lignified walls almost completely 
obliterating the cell cavity. 
The ground-tissue is composed of hexagonal parenchyma, but the cells 
of the outer layers are elongated and strongly sclerosed, affording to the 
leaf-stalk the stiffness mentioned above. The walls of these mechanical 
cells are bright red in colour and minutely pitted ; the stratification is 
particularly well marked. The cells remain living for some considerable 
time, protoplasm and large nuclei being constantly found to be present. 
The somewhat small intercellular spaces were carefully examined for any 
indications of the cellulose pegs or outgrowths described by Boodle and 
others as occurring in similar positions in certain ferns, but such structures 
seemed to be quite absent. 
The petiolar bundle is small and surrounded by an extremely well- 
marked endodermis, the outer tangential walls of which are strongly 
sclerosed and of great thickness. The inner tangential and radial walls 
