Differentiation in the Stem of the Sunflower . 505 
very base the somewhat greater thickness of secondary wood than corre- 
sponds to mere age can be interpreted as an accommodation phenomenon 
appropriate to the transition from the tangentially expanding stem to the 
pithless, radially thickening root. 
As regards (3), if upper and lower regions are compared at corresponding 
stages of their ontogeny, there is no clear evidence of an increasing propor- 
tion of parenchyma upwards. Moreover, in any given region, after primary 
expansion is complete, the proportion of functional parenchyma diminishes 
because of the increasing amount of secondary wood and the collapse of the 
pith. The real differences, therefore, between the upper and lower regions 
of the stem of Helianthus annuus are reduced to (a) a difference of scale 
and ( b ) a'difference of age and ontogenetic stage. Neither of these differ- 
ences is fundamentally morphological. The morphological basis is the same 
throughout. Nor does either of these differences appear to justify an appli- 
cation of the theory of recapitulation. All that can be said is that, just as 
the Sunflower is shorter lived than a tree, so the upper part of a Sunflower 
is shorter lived than the base. All that remains as anatomical evidence for 
the derivation of this herbaceous stem from a woody form is simply the 
presence of a cambium which gives rise to woody tissues similar to those 
which characterize woody stems. 
The increase of scale in the upper part of the shoot calls for some 
further consideration. Closely correlated with it is the secondary growth 
in thickness of the parts below by a method which allows of a rapid increase 
in diameter and at the same time is economical. Thus the necessary 
increase in rigidity keeps pace with the expansion above without drawing 
as heavily on the available food supplies as if the same increase in strength 
were secured entirely by radial growth, like that of a woody twig. A larger 
proportion of the food is therefore available for the young developing 
organs. The increasingly vigorous primary growth of these, which is 
reflected in the increase of scale, must involve a great expenditure of food 
material. Translocation must be proportionately efficient. Since Sachs’s 
experiments Helianthus annuus has been well known as a plant which both 
assimilates and translocates the products of assimilation with remarkable 
rapidity. Recently Willstatter and Stoll 1 have demonstrated that this 
species, along with certain other vigorous herbaceous plants, owes its 
capacity for rapid assimilation, not to a greater content of chlorophyll, but 
to greater efficiency of the chloroplasts themselves. It may be suggested 
that for the maintenance of this efficiency translocation must be correspond- 
ingly efficient so that accumulation of products is avoided. 2 
1 Untersuchungen liber die Assimilation der Kohlensaure, Berlin, 1918, p. 85, &c. 
2 Further light may be expected on this point from Briggs, Kidd, and West’s quantitative 
studies of growth in Helianthus ; cf., for instance, their reference to the possibility that the utiliza- 
tion of assimilable material, governed by temperature, might control assimilation. Annals of 
Applied Biology, vii, 1920, p. 217. 
N n 
