580 Hatfield. — Anatomy of the Seedling and 
8. The swollen hypocotyledonary region of the root, in which all the 
earlier-formed xylem has become disrupted and doubled up, as the result 
of the extreme distension of this region. In view of this distortion of the 
tissues, it seems probable that conduction is maintained only via the newly- 
formed secondary elements which border the whole conducting cylinder 
(Text-fig. 8 , h). 
9. An extremely long root of normal diarch structure, which develops 
few branches at this stage ; this is secondarily thickened in the normal way 
(Text-fig. 8, 1). 
III. Theoretical Considerations. 
(i) The Anomalies of the P arenchymatous System and the Phenomenon 
of ' Girdling ’. 
The form of vascular supply which characterizes the mature foliage leaf 
would seem to be the result of two factors. In the young leaf a broad 
decurrent base of insertion necessitates the derivation of the vascular supply 
from points widely separated on the vascular cylinder. The extreme 
telescoping of the axis, and the complete absence of internodes, has as 
a result that those strands of the leaf-trace which are derived from regions 
of the vascular axis removed by nearly 180° from the leaf midrib pursue 
tangential and nearly horizontal courses into the leaf margins. 
This condition, established near the stem apex, is subsequently 
modified by the activity of the specialized ‘growth layer’ or cambium, 
which occurs in the sub-apical region. The activity of this layer results 
in the interpolation of a considerable amount of parenchymatous tissue 
between the point of origin of the lateral leaf-trace and the tangential 
portion of its course. As a result of this, the tangential portion — 
‘girdle’ — of the trace is carried towards the periphery of the stem, 
while its radial connexion with the stele becomes longer. The leaf-supply 
characteristic of the mature leaf is thus established. The structure of the 
leaf-traces seems to harmonize with this view. They are composed of 
open, scalariform vessels, as opposed to the multiseriate pitted vessels of the 
stem,' and this retention of a primitive feature lost in the adult stem may 
be interpreted as a device for keeping such traces elastic as the girth of the 
stem increases (cf. Worsdell J ). 
It has, further, been shown that the earlier-formed wood vessels of 
these leaf-traces are born apart in the mature condition. This is probably 
best interpreted as a result of the stretching strain imposed on their tissues. 
If this view is the correct one, the ‘ girdle ’ is no new phenomenon whose 
origin is ‘a very obscure problem’. 2 Nor is it true that ‘traces of the 
youngest leaves at first ascend in an almost perpendicular direction, but, 
1 Worsdell : loc. cit. 
2 Coulter and Chamberlain : Morphology of Gymnosperms, p. 103. 
