374 Chandler . — On the Arrangement of the Vascular Strands 
observation (Fig. 51). At this stage preparations are made for the exit of 
the next leaf-trace, which is formed opposite the previous one and exactly 
above the preceding root. The strand of fundamental-tissue gradually 
assumes a more central position in the phloem, and a few sections higher 
up has increased in size and taken up a position near the outgoing leaf- 
trace. The previous leaf-gap is not yet closed. As will be seen from 
Figs. 54-57, the leaf-trace when complete is a simple concentric strand 
surrounded by an endodermis, the inner half of which is derived from 
the outer part of the endodermis surrounding the central strand of funda- 
mental-tissue, and not from the external endodermis by a process of 
‘ nipping in/ as is so generally the case. 
As soon as the leaf-trace is completed, we have the inner half of the 
inner endodermis continuous with the outer endodermis round the horns of 
the xylem arc, the internal strand of fundamental tissue thus effecting 
a junction with the main mass of ground-parenchyma (Figs. 55—57). 
The previous leaf-gap then gradually closes, and finally we have 
an arc of xylem including a patch of phloem, which is encroached upon at 
the leaf-gap by the ground-tissue. The latter is limited at this region 
by an endodermis having quite a different origin from a mere $ invagination/ 
As we pass upwards, the encroachment of the ground-tissue upon the central 
phloem becomes less and less, and finally the endodermis merely bridges 
the gap. 
The sixth leaf- trace is formed in exactly the same way as the previous 
one, with the exception that the internal strand of fundamental-tissue 
is, from the first, in close proximity to the leaf-trace with which it is to be 
finally associated. After the trace is completed, however, the ‘ invagination * 
of the ground-tissue is not diminished as before, but increased, the more so 
as the preparations for the next leaf-trace advance (Figs. 61, 62). This leaf- 
trace differs from the preceding in being double and not single, and 
consequently different processes are involved in its exit. 
In Fig. 60 we see the xylem arc broken at x in preparation for the 
trace, the two portions being separated by a band of phloem. This break 
is in reality the unclosed fifth leaf-gap, which is never fully repaired. 
In Figs. 61 and 62 the fundamental-tissue has advanced upon the medul- 
lary or internal phloem, which finally becomes relegated to a position 
merely lining the internal surface of the tracheary strand. This advance of 
the ground-parenchyma is complete, as it were, in Fig. 63, and we have two 
strands of vascular tissue separated from one another by the conspicuous 
fundamental-tissue, and each surrounded by phloem, pericycle, and 
endodermis. At a stage shown in Fig. 64, the two strands which will 
form the future trace are beginning to be nipped off, and it will be observed 
that the two horns of the xylem arc have approached one another, and that 
there is but a single fused endodermal layer between them. As the leaf- 
