506 Thoday. — On the Organization of Growth and 
It has been shown how at the node and elsewhere there is anastomosis 
of phloem, independently of the xylem, so that a minimum path to the 
shoot apex is secured for at any rate a part of the products entering at the 
node. Whether in this respect the Sunflower is exceptional or whether by 
any other structural features translocation is specially facilitated are ques- 
tions requiring comparative investigation . 1 
Whatever the answers to these questions may be, the fact remains that 
the rapid expansion of the shoot necessarily implies the rapid translocation 
of increasing supplies of food materials from the assimilating leaves. While 
expenditure of food increases with increase in the available supply, there 
seems no reason to expect increased storage. The emphasis laid by Jeffrey 
and by Sinnott and Bailey on the greater provision of storage tissue in herbs 
appears, therefore, to be exaggerated. 
Direct evidence on this question is incomplete, but it may be noted 
that although starch appears in abundance in the leaf of the Sunflower as 
a transitory reserve, it is absent from the stem, except in the endodermis, 
and little inulin crystallizes out in alcohol. 
It is also to be remembered that storage is not the only function proper 
to parenchyma, but that it plays a very important part in the mechanics of 
primary growth, especially in elongation. 
Secondary parenchyma . Jeffrey has laid special stress on the parenchy- 
matization of the secondary xylem of the foliar traces as providing 
additional storage tissue. Here again there is no direct evidence that this 
secondary parenchyma is in the Sunflower specially concerned with storage. 
In any case there are other points of view from which it may have 
significance. 
In the first place the secondary tissue in the leaf-trace bundle not only 
becomes more parenchymatous upwards, but. also diminishes in amount till 
at the node it is negligible. The mechanical aspect of this gradation is 
probably of considerable importance. If the cambium were radially active 
immediately below the node the primary xylem in the stem would be 
separated from the leaf base, and in the outgoing portion the vessels would 
be put out of action. Instead of this, the strain is evenly distributed and the 
primary xylem strand slopes gently outwards towards the point of depar- 
ture from the vascular zone. The strain is not, however, wholly removed. 
Occasionally the secondary parenchyma just outside the primary xylem is 
stretched, like the pith within, and the primary xylem is isolated from the 
later fibrous secondary xylem. In such case the presence of yielding 
parenchyma is clearly of mechanical advantage, as well as the collenchy- 
1 It is, of course, unlikely that the vigour of assimilation and growth depend wholly on 
structural features; e. g. Molisch (Mikrochemie der Pflanzen, 1921, p. 91) names the genus 
Helianthus among ‘nitrate plants’, and with this and other herbaceous genera named as rich in 
nitrates contrasts trees as generally poor in nitrates. 
