A Theory of the Double Leaf-Trace. 
79 
seedlings of Dicotyledons and of Gymnosperms for several years 
past. 1 
I.—Structure of Dicotyledonous Seedlings. 
The seedlings first examined were those of plants belonging to 
orders grouped according to Engler in the cohorts Ranales and 
Rhoeadales. 
The vascular system of the cotyledons, hypocotyl and root is 
very characteristic, and the mode of transition from cotyledons to 
root extremely simple, and with slight variation, and one or two 
exceptions, uniform throughout the genera and orders examined. 
The striking feature in the cotyledon is the presence of a 
peculiar V-shaped bundle, which is obviously composed of two very 
dc^nite halves, and has therefore been called a “double bundle.” 
The apex of the V is occupied by a common group of protoxylem, 
while at the extremity of each arm is seated a phloem group (see 
text-fig. la). 
DIARCH TYPE. 
Fig. I. Series showing transition features from cotyledons (a) through 
hypocotyl (b) to root (c). Protoxylem black; metaxylem hatched; phloem 
white. 
At the base of the cotyledon petiole the protoxylem may be in 
such a position in relation to the rest of the bundle that the point of 
the V is either directed inwards V or outwards A, or the V is 
flattened out into a secaut —•— with the protoxylem in the centre 
of the line. In any case the single protoxylem group from each 
cotyledon early assumes the external A position in the hypocotyl, 
and furnishes a pole of the diarch xylem-plate of the root. 
The alternating phloem groups of the root are derived by fusion 
of two phloem groups, one from each cotyledon. Thus the plane 
passing through the poles of the diarch root is the cotyledonary 
1 Tansley and Thomas. “ Root structure in the central cylinder 
of the hypocotyl,” New Phytologist, 1904. 
Tansley and Thomas. “ The Phylogenetic Value of the Vascular 
Structure of Spermophytic Hypocotyls,” Brit. Ass. Report, 
1906. 
