Holden and Bexon.—On the Seedling 
580 
cells deep, more than one layer of cells is involved in the subdivision to form 
cambiform elements (Fig. 28). We were unable to detect any indication of 
junction between the pericyclic cork cambium of the root and that arising 
sub-epidermally in the stem. 
(iii) The Vascular System of the Leaf and Petiole. 
The midrib, at the tip of the leaf, is represented by a small collateral 
strand which, as it is added to by lateral veins, gradually forms a shallow 
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Figs. 30-5. Diagrammatic figures illustrating the building up of the petiolar vascular system. 
Figs. 35 A, B, c show the common variants derived from Fig. 34. The bundles a and b are lateral 
bundles which may undergo displacement near the base of the lamina, and d and e are portions of the 
adaxial bar which may undergo similar alterations in position : the bundle x is one which is 
occasionally displaced at a higher level. 
gutter-shaped system of collateral bundles the concavity of which is adaxial 
(Figs. 30, 31). At a lower level, as further contributions from the lateral 
veins occur, it becomes ‘ C ’-shaped, as seen in transverse section (Fig. 32). 
The incurving of the free margins appears to be due to the displacement 
towards the adaxial side of some of the components from a higher level and, 
nearer the basal portion of the lamina where the largest lateral veins join 
the midrib, results in the formation of a closed ring of bundles (Figs. 33, 35 B). 
The adaxial portion of this closed ring is flattened and undergoes consider¬ 
able secondary thickening, so that it is sometimes difficult to determine the 
limits of the original components. Whilst the formation of the closed ring 
is characteristic of all the leaves examined, a number show from one to three 
small internal strands in addition. These may occasionally consist only of 
phloem (Fig. 51 a ), but are usually collateral and show an inverted orienta- 
