Differentiation in the Stem of the Sunflower. 501 
less in amount and at first consisted mainly of fibres. Only later was the 
formation of vessels resumed, and these were small. Comparison with 
bundle D, and with adjoining regions, where in the latest xylem the vessels 
diminish to a corresponding size, suggests that the plant is coming to the 
end of its growth (the inflorescence was already expanded), so that the 
cambium of bundle C never awakened to full secondary activity. In 
bundle B the cambium has been still less active ; it formed at first only 
Text-fig. 9. a-d. Median trace bundles at one level, belonging to leaves at four successive 
nodes above the plane of section, drawn from the section shown in Text-fig. 4 and Plate XVII, Fig. 5, 
illustrating the behaviour of the fascicular cambium at different distances from the leaves (see text). 
Wood fibres shaded. 
parenchyma and then added a few bands of fibres but no vessels. In 
bundle A there is least secondary tissue and that is almost wholly 
parenchymatous. 
Between corresponding parts of one leaf-trace bundle the contrast 
would be even greater. 
Latest phase. In older, lower internodes later stages in the ontogenetic 
sequence are to be found, but on a smaller scale. Throughout the leaf-trace 
bundle the cambium ultimately awakens to full secondary activity and 
