the Embryo and Seedling in the Gramineae. 203 
younger of the two seedlings very little tissue is lignified in this region, and 
this adds to the obscurity in structure. 
We have already remarked that the chief difference in vascular 
symmetry between the seedlings of Avena and Triticum , besides the 
absence of a mesocotyl in the latter, lies in the insertion of the coleoptile 
bundles. In Avena each enters between two lateral traces from the first 
leaf, and unites with both. The root-plates depend chiefly on traces from 
the second and third leaves. In Triticum the lateral traces of the first leaf 
go bodily into the root-plates, and the coleoptile traces enter the stele in the 
by-gap between those plates. The difference can be appreciated most easily 
comparing Diagram II in Text-fig. 16, p. 174, with II in Text-fig. 28. The 
younger seedlings of Triticum , in which the root-plates are not differentiated 
at that level, show the insertion of one lateral leaf-trace on either coleoptile 
bundle. 
This anatomical distinction is no doubt correlated with the single 
root-system of the Triticum seedling. In Avena the first leaf is supplied 
with water through the mesocotyl from the lower root-system, which 
includes the lower set of cauline roots as well as the primary root. The 
first leaf is therefore mainly connected with the traces which run down the 
mesocotyl. But in Triticum there is only one system of cauline roots. 
They correspond to the upper or nodal root-system of Avena. While the 
first leaf draws water from the primary root by its midrib, it must also be in 
direct connexion with the nodal roots through their insertion-plates. 
Hordeum vidgare , L. In external characters the seedling resembles that 
of Triticum , but the cauline roots are more numerous. In all the seedlings 
examined anatomically, the first leaf is still enclosed in the coleoptile. 
The scutellum has two main bundles symmetrically placed. Each 
resembles the single bundle of Triticum in being a trunk, built up of the 
slender branches which ramify in the apical region of the scutellum. When 
first formed, both trunks are nearly or quite amphivasal: lower down, the 
xylem in each becomes a ventral crescent. Just above their insertion on 
the axis the xylem groups are much extended laterally, and each is 
bisected by a group of protoxylem elements. 
The dual symmetry of the scutellum simplifies the ground-plan of its 
insertion. For when the two bundles run into the axis, the phloem of each 
turns outwards, and passes into the adjacent coleoptile trace. The greater 
part of the xylem of each bundle accompanies the phloem, but the inner 
xylem elements are in contact with each other, and go straight to the stele, 
where they meet other elements from the incoming coleoptile bundles. 
All this xylem turns downwards, and forms a massive strand within the 
stele for a short distance below the node. It is soon lost in the tangled 
anatomy of root-insertions. 
