164 
Sargant . — The Reconstruction of a Race of 
The protoxylem of the double trace is often completely external by 
the time it enters the stele of the hypocotyl, and the transition to a diarch 
root-stele is then very simple (Figs. 4-6), though commonly delayed for a 
short distance by the insertion of plumular traces (Fig. 5). These traces 
do not affect the symmetry of the stele in the hypocotyl and root. In 
Delphinium and Nigella , as in most seedlings with elongated hypocotyls 
among the Ranunculaceae, the stele becomes quite root-like high up in the 
hypocotyl. No further change is necessary as it passes into the primary 
root. In other families, however, the transition is often prolonged. 
In Nigella it is clear that the double structure belongs to the midrib 
only. The lateral traces when inserted on it do not affect its symmetry. 
By analogy we must suppose that the double structure of the trace in 
Delphinium also is due to the influence of the midrib. 
TETRARCH TYPE 
Figs. 10-T2. Tetrarch type of vascular symmetry, adapted from Miss Thomas’s diagram (88, 
p. 81) : 10, cotyledons with first leaf between them; 11, hypocotyl ; 12, primary root. 
In the extreme form of the tetrarch type, as described by Miss Thomas 
(88, p. 81), the midrib is represented in the petiole by two distinct bundles. 
As two laterals are also present, the petiole of each cotyledon contains four 
bundles, and these enter the hypocotyl as distinct traces. The stele of the 
hypocotyl receives eight cotyledonary traces. The two pairs which repre- 
sent the midrib-traces in such a form as Nigella behave exactly as in the 
diarch type. The protoxylem groups of each pair form a single protoxylem 
ray of the tetrarch root-stele. The right-hand lateral of one cotyledon 
joins with the adjacent lateral of the other to form a third protoxylem ray, 
and the left-hand laterals form a fourth in the same way (Figs. 10-12). 
This extreme case is less frequent than the symmetry described by 
Gerard in Althea rosea , a form also included within the tetrarch type of 
Mr. Tansley and Miss Thomas ( 87 , also 88, p. 80). The bundles from the 
blade approach each other very closely in the long petiole of the cotyledon 
