334 
Holden . — Observations on the 
led to the entry into the petiole of smaller veins of secondary rank which 
normally would fuse with the marginal or lateral veins within the cotyle- 
donary lamina. There is little doubt that the increase in the complexity of 
the petiolar vascular supply is to be correlated, in part at least, with the 
increase in the area of cross-section of that organ, this having become U- or 
C-shaped with the concavity on the adaxial side. One seedling of this 
type shows a curious anomaly in the arrangement of its petiolar vascular 
system, a presumably lateral strand lying in the same plane as, and imme- 
diately ventral to, the median strand. It retains this position throughout 
(Figs. 53 and 58-62), and the second pole of the diarch hypocotyl is 
produced by a concentration of the more distal lateral strands upon it, thus 
contrasting with the usual condition in which the concentration is bilateral 
and away from the middle line rather than towards it. There is also 
a further peculiarity in that the extreme marginal strand of one side is 
locally pseudoconcentric in character owing to the xylem being at that 
point completely surrounded by phloem (Fig. 59). From the stage in 
which the fusion of the lateral and marginal strands only occurs at the base 
of the petiole, further progress is inaugurated by this union being delayed 
until their entry into the hypocotyl. This is followed by the two becoming 
entirely independent (Figs. 81-83), and in one case the two strands of one 
side show delayed fusion, whilst on the other the marginal constituents 
alone take part in the formation of a root pole (Figs. 73-75). In such cases 
the strands representing the laminar laterals traverse the greater part of 
the hypocotyl, but ultimately their protoxylem disappears and their identity 
is lost in the xylem complex associated with the production of the root 
whorl to which reference has already been made. As long as the pro- 
toxylem persists it retains an endarch position, the only exception noted in 
the eight cases of this type being one in which it became mesarch towards 
its lower end. In one seedling both laterals are independent and persistent, 
and each forms a root pole so that a tetrarch condition is once more 
established (Figs. 84-90). 
The seedling in which this phenomenon occurred was very short and 
stout (Figs. 22 a, lib), but whether this had any bearing on the develop- 
ment of tetrarchy it is impossible to say. A condition of secondary triarchy 
would obviously arise in a similar way granted an asymmetrical inde- 
pendence and persistence of one of the lateral strands. The median strand 
of this particular plant is remarkable in assuming a pseudoconcentric 
structure at the base of the petiole, though this is not persistent (Figs. 84- 
86), and a similar feature also characterizes the two lateral and to a certain 
extent the marginal strands immediately before and subsequent to their 
entry into the hypocotyl (Figs. 85-86). There is a distinct similarity between 
these strands and the ‘ Zwischenstrange ’ recorded by Dodel ( 5 ) in PJiaseolus 
spp. and by Compton ( 2 ) in P. Herncindesii , Abrus precatorius , and in 
