168 Sargant and Arber .— The Comparative Morphology of 
downwards, doubling on itself in such a way that in transverse section two 
double bundles appear on opposite sides of the sheath. When the down¬ 
ward bundles approach the first node they leave their companions and 
enter the stele among the plumular traces. 
This imaginary type X resembles Elettaria in possessing two distinct 
bundles in the cotyledon, which make an acute angle in the sheath before 
entering the stele of the hypocotyl from opposite sides. To derive it from 
Elettaria the stalk of the cotyledon must be inserted on the axis rather 
than on the sheath ; both bundles must approach the top of the sheath 
before they turn down, and they must double on themselves so closely that 
the upward and downward segments of 
each bundle are in contact throughout 
the upper part of the sheath. 
For such close doubling of a bundle 
there is a precedent in the seedling of 
Tigridia (Text-fig. 8, p. 168). A single 
bundle enters the sheath from the co¬ 
tyledon, travels up the dorsal spine of the 
sheath, and doubles on itself near the top. 
It appears in transverse section as a double 
bundle with two external xylem groups, 
dorsal and ventral, for some distance 
below the turn (PL X, Fig. 15). This 
unusual orientation of xylem and phloem 
depends on the median position of the 
bent bundle in the Tigridia sheath, and 
would not occur in the lateral bundles of 
the X sheath, however closely each might 
bend on itself. 
To derive the vascular skeleton of 
the Avena type from that of X, four 
modifications are necessary. (1) The 
two bundles of the cotyledon must unite with each other throughout 
their course in sucker and stalk. Examples of such union are not 
uncommon among Monocotyledons. 1 The distinct bundles of one species 
may be represented by a double bundle in another species within the same 
genus. (2) The stalk of the cotyledon must unite with the hypocotyl 
so completely that the sucker appears sessile on the axis at a level below 
that of the first node. Thus a mesocotyl is formed by fusion of the 
cotyledonary stalk with part of the hypocotyl. 
1 Sargant, E. (’ 03 ) : A Theory of the Origin of Monocotyledons founded on the Structure ot 
their Seedlings. Ann. of Bot., vol. xvii, pp. 20-1, 1903. Scilla sibirica has two separate bundles 
in the cotyledon, and S. peruviana a double bundle. Cf. also the case of Colchictwi autulnnale 
mentioned below, p. 218. 
Midrib of 
first leaf 
Text-fig. 8. Diagram of bundles in 
sucker and sheath of Tigridia. 
