510 Thoday. — On the Stem of the Sunflower. 
upper region represents an earlier phase of ontogeny than the woody base. 
The validity of Jeffrey’s application of the theory of recapitulation to such 
differences is questioned. 
8. The stems of small starved plants form very little secondary tissue 
even at the base, and their structure suggests comparison with smaller and 
shorter-lived annuals. 
9. Various interpretations of the facts are discussed. 
EXPLANATION OF PLATE XVII. 
Illustrating Professor Thoday’s paper on the Stem of the Sunflower. 
The figures are reproduced from photographs of transverse sections. 
Fig. 1. One of the two principal median leaf-trace bundles at the middle of the first epicotylar 
internode of a young plant, showing the continuity of the rows of elements of the primary xylem 
through the cambial zone. 
Figs. 2 a - 2 f Sections of an old stem photographed about natural size in oblique illumination, 
with a dark background to show up the wood. 
a , hypocotyl, about 1.5 cms. below the cotyledonary node. 
b-e, first epicotylar internode : b , just above the cotyledonary node ; c , 1 cm., d, 2 cm., e } 8 cm. 
above the node. 
The series a-e illustrates the increasing distension of pith and diminishing thickness of the 
woody zone upwards (see p. 491). 
f, at a higher level, in the region of transition to two-fifths phyllotaxy. 
Fig. 3. Same as 2 b, x 4. Base of first epicotylar internode, showing continuous zone of w'ood. 
The older wood, next the pith, is not continuous, and this together with the distension of the pith 
is evidence of an earlier tangential phase of growth. This has now been superseded by the later 
phase in which fascicular and interfascicular cambium behave uniformly and growth is radial, as in 
a woody twig. 
Fig. 4. Same as 2 e, x 4. Upper part of same internode. Tangential growth is about at an 
end. The principal bundles are separated by broad wedges of secondary wood in which the 
divergence of the secondary medullary rays is evidence of tangential growth. The distended pith 
shows clear signs of strain. 
Fig. 5. Same as 2 x 4. The number of bundles is larger, but tangential growth is still 
exhibited between the principal ones. The pith has given way in the middle and the outer intact 
zone shows evidence of strain. 
Fig. 6. A part of the same section more highly magnified. The two bundles near the ends of 
the photograph are the median and one of the lateral trace bundles belonging to the leaf at the 
second node above the section. They show the parenchymatous nature of the secondary xylem 
formed at first by the cambium in the upper part of the leaf-trace bundles. 
