1 67 
the Embryo and Seedling in the Gramineae . 
and also M, the midrib trace from the first leaf. The coleoptile bundles 
are bisecte 4 by the long axis at right angles to this. They enter the stele 
at opposite sides, and the adjacent plumular traces are inserted on them. 
Then each coleoptile trace divides. The phloem groups separate, one 
turning outwards and carrying some xylem elements with it, while the 
other continues within the stele, and meets the corresponding branch from 
the opposite coleoptile trace there. These united branches turn downwards, 
and constitute the bundle x within the stele (Text-figs. 6, p. 166, and 16, p. 174). 
The outward branches meet too, but outside the stele. Together they build 
up the scutellum trace. 1 
The direct connexion between scutellum trace and coleoptile bundles 
is the most striking feature in the Arena type, and we have therefore 
searched for some special relation between sucker and sheath in the 
vascular structure of other monocotyledonous seedlings. The comparison 
which we think most instructive is with the seedling of Elettaria car da- 
momum , one of the Zingiberaceae. In this species the sucker of the 
cotyledon remains within the seed, and is connected with the axis by 
a fairly long stalk. The sheath is partly above the insertion of the stalk, 
and partly below it. The stalk runs up to its insertion, and in that neigh¬ 
bourhood is commonly almost parallel with sheath and axis (Text-fig. 5). 
The two bundles of the cotyledon are symmetrically placed within the 
stalk, the xylem of each being internal. On entering the sheath, however, 
they diverge to right and left. One travels upwards, and approaches the 
apex of the sheath before turning sharply down. The other turns down 
almost as soon as it enters the sheath (Text-fig. 5). In the end they enter 
the stele of the hypocotyl from opposite sides. The traces which separate 
them are plumular. 
We have constructed an imaginary vascular skeleton to link the 
symmetry of the Arena type with that of Elettaria (Text-fig. 7). The 
stalk of the cotyledon contains two independent bundles. These enter 
the axis at the first node, where each turns outwards into the cotyledonary 
sheath, and then upwards within it. Near the apex each turns sharply 
1 The vascular skeleton described above is that of the Avena type, generalized from Avena and 
from other genera resembling it in seedling structure, notably from Zizania. It differs in one 
important respect from the vascular skeleton of Avena itself. In describing the latter we shall show 
later on that the scutellum trace is directly connected with the stele by a massive xylem arch (p. 178). 
We have come to the conclusion that this feature is adaptive, not primitive ; perhaps a device for 
supplying the scutellum with water from the lower roots in the most direct way. For in Zizania, 
although, as usual in an aquatic species, the whole xylem skeleton is very much reduced, it is still 
identical with that of Avena except in this one respect. No general reduction of xylem elements 
would account for the complete disappearance of a massive xylem arch between scutellum and stele. 
But the submerged scutellum of Zizania needs no short cut to secure an adequate supply of water, 
and this species has no lower system of cauline roots. 
There are some independent grounds for considering Zizania as a genus with primitive features, 
particularly its floral structure. 
