SEEDLING STRUCTURE IN THE LEGUMINOS JA. 107 
genetie relation to these types of those cases in which the plumular traces 
contribute primary xylem to the hypocotyl and root. Is the plumular trace 
to be regarded as a modified intercotyledonary strand, or is it a new structure 
superposed upon the cotyledonary vascular system ? 
We shall see that triarchy, which has already proved so useful a key to 
phylogeny, enables us partially to answer this question also in a rather 
unexpected fashion. 
Two well-marked types of triarchy occur in the Papilionatze. In the first 
or epigeal type, which we have already discussed, all three protoxylems of 
the root are essentially cotyledonary. In the second type, characteristic 
of the hypogeal climbing Viciex, two protoxylems enter the cotyledons 
(which are borne at an angle of about 120° from one another), and the 
third passes up into the first leaf. Can we regard the intercotyledonary 
xylem as homologous in the two cases? It seems quite clear that we must 
not do so, for the following reason :— 
A comparison between the two types reveals the curious fact that in the 
epigeal Hedysarez, Galegeæ, Ke., the first leaf is borne on the opposite side 
of the axis to that on which it is borne in the hypogeal Viciew. This 
becomes clear from a study of the manner in which the embryo is folded in 
the seed. The radicle is accumbent in both cases, the axis of the embryo 
being bent. In the Vicieæ the first plumular leaf is borne on the convex side 
of the bend ; whereas in the epigeal species it is borne on the concave side. 
This is shown also after germination, for the edges of the cotyledons which 
were pressed against the radicle and hilum in the seed are often marked by 
a sinus or straightness which is absent from the external edges. In the 
hypogeal Viciew the first leaf is borne on the side away from this sinus ; in 
the epigeal Galegez, Hedysarew, &c., on the side towards it. 
It follows that the lateral root-pole which is present in the triarch Vicieæ 
is on the opposite side of the axis to that present in the epigeal cases of 
triarchy ; and therefore it is out of the question that one type can have been 
derived phylogenetically from the other by a change in the róle of the inter- 
cotyledonary xylem *. 
* The transition from one type of seedling to the other was very possibly effected 
through the mediation of forms in which the first two leaves are opposite. The Viciez 
type may have been derived from such a condition as is found in the hypogeal Phaseoleze 
where the primordial leaves are opposite; such a plant as Abrus precatorius, an anomalous 
member of the Viciez, exhibiting the Phaseoleew type of seedling. It is noteworthy, too, 
that forms with opposite first leaves occur in the Galegeze (Indigofera, Psoralea) and in the 
Hedysarez (Desmodium). Moreover, in triarch Lotus corniculatus (see p. 103, footnote) the 
first leaf arises (? always) on the same side as in the Viciem, 7. e., on the opposite side to 
the intercotyledonary xylem; but it does not contribute to the primary structure of the 
hypocotyl. An alternative hypothesis would be to derive the cotyledons and first leaf of 
the Viciez from a tricotyledonous type; in an example of this kind in Lotus corniculatus 
