SEEDLING STRUCTURE IN THE LEGUMINOS.E. 103 
being correlated with the early production of foliage-leaves. Casuarina and 
Cassytha, for instance, are considered to have ** escaped anatomical reduction 
[from tetrarchy to diarehy] on account of the insignificance of the plumular 
traces ” (Thomas, 1907, p. 86). A replacement of primary by secondary xylem 
is thus supposed to have occurred in the change from tetrarchy to diarchy 
the increase in the amount of secondary tissue being due to accelerated 
epicotyledonary development. 
When we attempt to apply this theory to cases of triarchy we are met by 
a serious difficulty. If it be granted that triarchy is an intermediate stage 
in the course of reduction from tetrarchy to diarchy (and this seems 
inevitable, once admitting the reduction hypothesis), we should expect on Miss 
Sargant's theory that the first pole to be lost would be the one opposite the 
greatest development of secondary xylem, 7. е., the one on the side of the 
first leaf. Now the reverse of this proves to be the case in almost all 
the triarch epigeal Leguminose examined *. The lateral protoxylem to be 
developed in the hypocotyledonary axis in cases of triarchy is the one on the 
side of which the first foliage leaf arises; and in those triarch seedlings 
in which a rudimentary fourth xylem appears, this is on the side of the 
second leaf. 
It is clear that these facts cannot be explained on Miss Sargant’s reduction 
theory of the replacement of primary by secondary xylem in the inter- 
cotyledonary plane in response to accelerated plumular growth. On the 
contrary, they strongly support the alternative theory, by which the plumular 
trace and the lateral root xylem are regarded as supplementary ; the relation 
between them is seen not to be one of supplantation, but one of joint 
participation in the early formation of a continuous conducting channel from 
root to leaf. 
If it be true that the plumular traces enter into a very close relation with 
the intercotyledonary root-poles we ought never to find triarehy in cases in 
which the first two plumular leaves are opposite (though the reverse 
proposition is not equally true). An examination of all the cases of triarchy 
occurring in the Papilionate shows that in every known instance the first 
two leaves are alternate f. This is specially noteworthy in the Galegew and 
Hedys: arem, for here we find certain genera with opposite primordial leaves 
and tetrarchy (Indigofera, Psor alea, Desmodium) ; others with alternate 
leaves and triarchy (Astragalus, Caragana, Hippocrepis). 
* In some triarch seedlings more than one epicotyledonary axis arises at the node 
(e. g. Scorpiurus sulcata), but one is stronger than the others. 
t Lotus corniculatus is exceptional in that the first internode is much elongated (a feature 
mentioned by De Candolle (1825, p. 89) in other Lotus spp.) and the first two leaves quite 
close together. In this species the first leaf trace appears (? always) on the side opposite to 
the third xylem, this being a definite exception to the rule. 
