496 T ho day, — On the Organization of Growth and 
with other incoming strands on both sides of the incoming bundle. Thus 
a phloem plexus is formed on each side, in close association with the 
phloem of the adjoining bundles which communicate with the upper 
expanding part of the shoot and later form part of the secondary synthetic 
strands . 1 
It is not only at the node, however, that communication is established 
laterally between the principal phloem strands. Anastomosing strands are 
frequent between all the bundles, in the internodes as well as the nodes, so 
that the phloem in its lateral continuity stands in marked contrast to the 
xylem. 
Ontogeny of the leaf -trace handle. As already observed, the leaf-trace 
bundles in the upper part of their course stand out in sharp contrast with 
the synthetic strands, not merely by reason of their isolation by flanking 
parenchyma, but also because the fascicular cambium behaves differently. 
Not only is it relatively inactive, but, as Jeffrey and his collaborators have 
observed in this and other herbaceous stems, the secondary xylem that it 
forms is for a time largely, if not wholly, parenchymatous. 
Before considering the possible significance of these peculiarities, it is 
necessary to complete the picture of the ontogeny of the stem by adding to 
the account of the later stages already given a description of the earlier 
stages, with special reference to the ontogeny of the individual leaf-trace 
bundle. 
The primary xylem is characterized by rows of vessels, mostly spiral, 
separated by bands of parenchyma two or more cells in width, and often 
diverging in a fan-like manner towards the pith. The typical secondary 
wood contrasts sharply with the primary, and consists of fibres and large 
pitted vessels, with medullary rays. But, notwithstanding the histological 
contrast, the rows of elements can often be traced continuously right 
through the primary and secondary xylem into the cambial zone. This 
continuity is only obscured where in the secondary wood the growth of 
large vessels has led to distortion, or where in the protoxylem the oldest 
vessels have collapsed and adjacent cells have encroached upon them. It 
is often clearest in the upper parts of the bundle, where the secondary 
xylem is at first largely parenchymatous and not distorted by vessels 
(Plate XVII, Fig. i). 
Examination of the early stages of development of the bundles confirms 
the inference that the primary and secondary xylem are formed by the 
activity of the same cambium and are only distinguishable histologically. 
If a vigorous plant is selected while it still shows no sign of the 
inflorescence, but is already producing a succession of large leaves, trans- 
1 In the case figured the axillary growing-point was latent and no vascular supply to it was 
even foreshadowed. Where an axillary bud is developed its vascular cylinder opens inwards and 
joins the vascular zone of the stem above the median leaf-trace bundle. 
