Differentiation in the Stem of the Sunflower . 499 
the centre. Cambial activity also extends along the inner margin and 
additional rows of vessels appear. 
At a later stage the cambium often adds also to the phloem, but it is 
difficult to tell how much of the phloem of a mature bundle has originated 
from it. 
The characteristic fan-like divergence of the rows of vessels towards 
the pith is the resultant of several factors, and varies in degree. The 
cambium is more or less curved ab initio , and the rows of elements tend to 
be at right angles to it. The curvature becomes more obvious as the 
cambium, following the procambial crescent, extends laterally. Later, the 
expansion of the fibres and the formation of additional phloem by 
the cambium diminish its curvature. On the other hand, the cells 
between the rows of vessels continue to grow and so widen the spaces 
between them and maintain the divergence. At the same time the 
surrounding parenchyma is growing, and mutual adjustment between this 
and the bundle, which bulges into the pith within and into the cortex 
without, is obviously necessary. The divergence is most pronounced 
where the bundles reach a large size and the general primary expansion 
is most vigorous. Towards the base of the stem, where the bundles are 
small and growth generally less vigorous, the rows of vessels are often 
nearly parallel (cf. Plate XVII, Fig. 1 ). 
The leaf -trace bundle at different levels . At a given level the bundles 
that belong to the next node are the first to appear and are precocious in 
their development. The trace bundles of younger leaves are successively 
interpolated and their development shows a corresponding lag. When the 
primary growth comes to an end the trace bundles of two or three leaves 
are already full grown. Further growth in the others is secondary in 
character. 
The structure of different regions of an individual trace bundle of a full- 
grown leaf varies accordingly. For two or three internodes (in the upper 
part of the stem) the xylem is more or less constant in form and size and 
consists entirely of primary tissue. Most of the vessels are spiral or 
annular, or partly annular and partly spiral. The last one or two in each 
row show reticulations or include an occasional pitted vessel. Farther 
down the amount of primary xylem diminishes and secondary xylem is 
associated with it in increasing amount (Text-fig. 8). It may be inferred 
that in the first two or three internodes the trace bundle reaches practically 
its full size while growth in length is still proceeding and the surrounding 
parenchyma is still growing actively ; and further that in the uppermost 
internode it is not only precocious but reaches its full size before the 
primary growth of the rest of the tissues is complete. 
So long as growth in length continues, vessels put out of action by 
elongation are doubtless replaced, and the last vessels are such as could be 
