396 Chandler . — On the Arrangement of the Vascular Strands 
in which the sieve-tubes take up a central position. The pericycle is com- 
posed of relatively large cells, and is succeeded by an obvious but not 
particularly well-differentiated endodermis. 
The third and fourth leaf-traces are formed in the same manner as the 
second, the external and internal phloem becoming continuous at the gaps, 
which are merely bridged by the endodermis. The pericycle gradually 
becomes two or three layers in thickness. After the exit of about the 
fourth trace, preparations are made for the change of the leaf-strand 
from a single to a double character. The change is effected in the usual 
way, viz., by the breaking of the curved xylem band into two before the 
closure of the previous gap, followed by a nipping off of a concentric strand 
from each of the vascular horns so formed. The two leaf-strands, however, 
unite immediately they leave the cauline system, so that the actual trace is 
single as before. At the level of the fifth or sixth leaf-trace we have the 
differentiation of a ground-tissue pocket of the usual type, a single cell, 
often containing tannin or mucilage, and surrounded by a well-marked 
endodermis, appearing in the central phloem. A little higher up, this 
internal endodermis surrounds a small group of ground-parenchyma cells, 
which become continuous with the main mass of ground-tissue at the next 
leaf-gap. At other times the endodermis appears immediately after the 
exit of a leaf-trace, and becomes continuous with the outer layer before 
the corresponding gap has been closed. In both cases the net result is the 
same, viz., the separation of the cauline vascular system into two concentric 
strands each surrounded by an endodermis (Fig. 82). 
Later changes are represented in the diagrams. One of the two 
cauline strands divides into two parts for the insertion of the double 
leaf-trace, and one part immediately joins with the other main cauline 
strand, thus closing the previous leaf-gap. The vascular system still con- 
sists, therefore, of two concentric strands (Figs. 83, 84). The leaf-trace is 
then given off ; as before, it is double at its origin, but enters the leaf 
as a single vascular rod. Figs. 86-88 illustrate the closing of the previous 
gap and the differentiation of the next leaf-trace. At the level of about 
the ninth leaf, the splitting of the cauline strand and the subsequent forma- 
tion of the trace takes place before the closure of the previous gap, i. e. the 
gaps ‘ overlap,’ and we consequently have the appearance of three cauline 
strands (Figs. 89, 90). 
Subsequent elaboration need be described in no great detail. By the 
repeated overlapping of the gaps, the number of cauline strands at succeed- 
ing levels is gradually increased, and the dictyostelic structure of the mature 
plant is soon reached. From about the eighth leaf onwards, the two 
strands of the leaf-trace remain separate through increasingly longer 
intervals until they enter the petiole as two, or in the later leaves, more 
than two, separate strands. 
