Differentiation in the Stem of the Sunflower . 509 
other woody plants is sufficient to make the cambial origin of the whole of 
the xylem in them also highly probable. 
Mrs. Arber’s studies of Monocotyledons, 1 on the one hand, and 
Chauveaud and, more recently, Lenoir’s investigations of seedling ontogeny 2 
on the other, point largely in the same direction. But on this question 
more definite data are desirable. 
Summary. 
1. The primary and secondary growth and differentiation of the stem 
of Helianthus annuus are described. The principal method employed was 
to compare the dimensions and structure of corresponding regions of 
different age and size. 
2. In the primary differentiation of the leaf-trace bundle the small pro- 
cambial strand is early distinguishable into an outer group of fibre initials 
and an inner procambial crescent which grows laterally. The whole of the 
xylem originates from a cambium which arises very near the inner margin 
of the crescent and extends tangentially in both directions pari passu with 
it. The bulk of the procambial crescent differentiates as phloem, beginning 
in the median region while the flanks are still meristematic and growing 
actively. 
3. The primary growth of the stem is illustrated, including the inter- 
polation of new bundles in the widening intervals between the precocious 
primary bundles. The transverse growth of a leaf-trace bundle is correlated 
with that of the leaf and not with that of the internode as a whole. 
4. At the node the entering leaf-trace bundles are accompanied by 
strands of phloem which connect laterally with the phloem of adjoining 
bundles. Anastomosis of phloem strands is also frequent elsewhere. 
5. Secondary growth in thickness is at first due to the activity of the 
cambium in the ‘ synthetic traces 5 that communicate directly with the upper 
still expanding leaves. This activity is in parts tangential as well as radial. 
Extensions of the cambium are also formed in the adjoining medullary rays, 
which widen to accommodate them. As a result of this tangential growth 
in the cambial region the pith is distended and often becomes hollow. 
6. The cambium in the upper part of the mature leaf-trace bundles 
is meanwhile inactive and only gradually resumes active growth. The 
secondary xylem is at first largely parenchymatous. Towards the base of 
the stem a later stage of ontogeny is reached in which the fascicular cambium 
forms secondary wood in no way differing from that formed elsewhere. 
7. The mode of growth, both primary and secondary, is the same from 
the basal internode throughout the stem. The differences are due to the 
larger scale of the primary tissues in the upper part and to the fact that the 
1 Ann. Bot., xxxvi, p. 251, 1922, and other papers there cited. 
2 Loc. cit. 
