On the Organization of Growth and Differentiation 
in the Stem of the Sunflower. 
BY 
D. THODAY, M.A., 
Harry Bolus Professor of Botany in the University of Cape Town. 
With Plate XVII and ten Figures in the Text. 
T HIS paper is the outcome of an inquiry into the meaning of the 
irregularity of the zone of secondary wood in the old stem of 
Helianthus annuus , which in this re- 
spect stands in marked contrast to the 
stems of most woody plants . 1 
The stem of a young Sunflower 
plant has the usual ring of vascular 
bundles, bulging a little in comformity 
with the prominence of the leaf-bases, 
but otherwise regular (Text-fig. 2). 
The medullary rays are not very wide, 
so that the arrangement of the tissues 
is not conspicuously different from 
that in young twigs of a multifascicu- 
late type of structure. It is during 
growth in thickness that the contrast 
develops. 
In the secondary growth of most 
woody twigs, secondary tissue is added 
more or less regularly and uniformly by 
radial activity of the cambium. The 
primary xylem and the pith appear to 
remain unaffected by the mechanical 
tissues. 
Text-fig. i. Transverse section through 
the middle of the third internode of a stem 
over a centimetre in diameter, showing the 
irregular formation of secondary wood. /, /', 
median and i, i , i', i\ lateral trace bundles 
of leaves at the node above ; //, //', median 
trace bundles at the next (fourth) node. 
Fibres are shown black. 
Text-fig. 2. (Inset in Text-fig. 1 on 
same scale.) Same region of a young stem. 
disturbances that strain the outer 
1 Compare Thoday, Botany, Cambridge Univ. Press, 1919, Fig. 35, p. 138. 
Annals of Botany, Vol. XXXVI. No. CXLIV. October, 1922.] 
