Differentiation in the Stem of the Sunflower . 491 
inactive. Evidences of strain in the inner part of the rays show that it is 
in the cambial region that the tangential growth has occurred. 
Accommodation phenomena . A full understanding of the unequal 
distribution of tangential growth involves a knowledge of the course of the 
bundles and secondary wood-sectors in adjacent parts of the stem, for 
accommodation is of course mechanically necessary. The widening of the 
rays flanking the principal bundles is probably in part an accommodation 
phenomenon of this kind, correlated with the growth of wood-sectors above 
the outgrowing leaf-trace bundles. 
Other examples are found at the base of the stem. For instance, the 
median paired bundles of the cotyledonary traces become widely separated 
Text-fig. 3. Parts of Text-figs, i and 2 enlarged. The * radial’ lines in the wood show the 
direction of rows of elements formed by cambial cells. The broken lines in the pith represent lines 
of strain. Secondary wood shaded ; rows of vessels indicated in primary xylem ; fibres black ; 
cb, cambium. 
in conformity with the tangential growth of a massive wood-sector in the 
internode above. At a later stage the interval is bridged by secondary 
wood. 
Another example is afforded by the diminution of tangential growth 
downwards in the lowest epicotylar internode and the hypocotyl. As the 
root is approached the pith suffers less and less enlargement and tapers to 
a point in the transition region. This is a natural accommodation to the 
absence of pith in the root. There is a complementary increase in radial 
activity of the cambium down to the root, where tangential growth would 
obviously be unsuitable to the mechanical requirements. It is largely for 
this reason that the base of the stem is so much more woody, compared 
with parts a little higher, than mere difference of age would lead one to 
Mm2 
