497 
Differentiation in the Stem of the Sunflower . 
verse sections at short distances from the apex may legitimately be taken 
to represent, approximately, different stages of the ontogeny at one level 
in that region, for the size of the mature primary bundles of successive leaf- 
traces has become fairly constant. Text-fig. 6 represents corresponding 
portions of three such sections, including the principal leaf-trace, drawn to 
the same scale. It exhibits in a striking manner the great increase in size 
of the bundles, their progressive separation with the growth of the whole 
stem, and the interpolation of additional bundles in the widening intervals. 
In the growth of the individual bundles it is to be observed that the rows 
of vessels increase not only in length but in number. 
Still nearer the apex the bundles are represented by small groups of 
procambial cells separated by narrow radial bands of developing parenchyma. 
Text-fig. 6. Corresponding portions of three sections, each including the three bundles of 
a leaf-trace, at different levels near the apex of a vigorous plant not yet beginning to flower. 
Phloem and procambium shaded except in earliest stage. 
These groups increase in size and soon become distinguishable into an outer 
part, with the cell contents rather less dense, which ultimately forms the 
fibres, and an inner crescent of denser procambium. 
The differentiation of this procambium does not correspond to the 
current view of the development of a collateral vascular bundle, according 
to which the procambial cells begin to differentiate on the outside as 
protophloem and on the inside as protoxylem. In this stem, cambial 
activity begins very early on the inner margin of the procambial crescent, 
and the whole of the xylem is the product of this. Moreover, the pro- 
cambial crescent grows laterally, the cambium extending also along its 
inner margin. 
In the earliest stage of the differentiation of vessels clearly recognizable 
