142 
Sargant . — The Reconstruction of a Race of 
The leaf-traces of a Monocotyledon do not run parallel to the axis 
of the stem throughout their course, and the original arrangement in 
concentric circles is soon lost. Torsion of the trace on itself leads to 
irregular orientation, and even the collateral structure sometimes becomes 
concentric. The type of bundle called by Professor Jeffrey amphivasal 
( 46 , p. 315) is so common in the older stems that he figures it as the typical 
Monocotyledonous bundle (loc. cit., Fig. 1 13, H^H). In this form of concentric 
bundle the phloem is completely surrounded by xylem. Professor Queva 
points out that in the stem of Gloriosa concentric bundles are due to 
anastomoses of collateral traces with each other ( 65 , p. 87 and Figs. 125, 
126). This is certainly the case in the scutellum of Zea Mats (Sargant 
and Robertson, 74 , p. 121 and Figs. 15-18). In the lower part, where 
branches are absent, the main bundle is collateral. But in the upper part, 
where branches are inserted all round the main bundle, it becomes almost 
amphivasal. 
Thickening-rings are formed in the cortical parenchyma of certain 
Monocotyledonous trees, but they do not constitute a cambium in the 
Dicotyledonous sense. 
The numerous differences in detail which separate the stem-structure 
of Monocotyledons from that of Dicotyledons depend more or less directly 
on the presence or absence of an active cambium. Its presence secures 
a single ring of leaf-traces, the persistence of their collateral structure, and 
their uniform orientation. This is illustrated by the structure of Dico- 
tyledons in which the cambium has become inactive and is disappearing. 
In such forms we find not only the scattered arrangement of the traces 
(Plolm on Podophyllum , 45 , pp. 429-30. Cf. also Fig. 10 on p. 52 of 
Solereder, 81 ), but even occasional amphivasal structure (Worsdell, 90 , 
p. 600). It was a true instinct which led early botanists to lay so 
much stress on the presence of a cambium. The character it affords is 
also one of the most satisfactory to taxonomists, for, though some Dicoty- 
ledons have lost the cambium, it is not found as a really active tissue in 
any Monocotyledonous stem. 
The question really is, then, whether the Primitive Angiosperms 
possessed a cambium. Evidence of two kinds is available : comparison 
of the Angiospermous stem with that of Gymnosperms and Cryptogams, 
and comparison of the mature stem of Monocotyledons and Dicotyledons 
respectively with the seedling stem of the same class. 
A cambium is the rule among living Gymnosperms, and universal 
among the extinct forms yet investigated. Traces of its existence are very 
rarely found among living Vascular Cryptogams (e. g., see Cormack, 17 ). 
The absence of a cambium was formerly considered as a conclusive mark 
