Differentiation in the Stem of the Sunflower . 493 
shoot as a whole in a way that stands in marked contrast to the behaviour 
of the local leaf-traces. 
This contrast, which has been illustrated already for the base of the 
stem, is not peculiar to that region. Under sufficiently favourable con- 
ditions a considerable amount of secondary thickening may occur at higher 
D 
Text-fig. 4. Sections in the region of transition to two-fifths phyllotaxy of two stems of different 
age drawn to the same scale. Fibrous secondary xylem cross-hatched ; rows of vessels indicated in 
primary xylem ; fibres black. Secondary growth, tangential in character, has occurred between the 
two stages represented. In the earlier stage tangential growth had already resulted in the collapse 
of the pith in the centre. A, b, c, d, median trace bundles belonging to leaves at successive nodes 
above the older section (see Text-fig. 9) ; a, a , &c., corresponding lateral bundles. (Photographs of 
this section are reproduced in Plate XVII, Figs. 5 and 6.) In the younger section the chief bundles 
are numbered according to their node of exit — median trace bundles with roman, lateral with 
arabic numerals. 
levels, and the farther it proceeds the more clearly is the same mode of 
organization revealed. Text-fig. 4 illustrates this. It represents a section 
of the stem of a tall plant, which already bore a giant capitulum, taken 
through the region of transition to two-fifths phyllotaxy ; also inset and 
represented on the same scale for comparison is a section through the 
corresponding region of a younger plant. The secondary growth is of the 
same tangential character and is localized in the synthetic traces. Plate XVII, 
