106 MR. R. H. COMPTON : AN INVESTIGATION OF THE 
Leguminoee in which change of symmetry is recorded *, 19 show an 
increase in the number of protoxylems as the hypocotyl is ascended, 5 a 
decrease, and 2 an inerease followed by a decrease, It may be suggested 
that this preference for reduction below points to the existence of a greater 
conservative tendency at the cotyledonary node than elsewhere in the axis ; 
or it may simply be an expression of the fact that the axis usually tapers 
rapidly downwards at the collet region (where the reduction so often 
occurs)—more rapidly than it tapers upwards at the summit of the 
hypocotyl. 
Whatever be the explanation, it seems unnecessary to ascribe a different 
significance to basipetal and acropetal modes of reduction: both alike are 
expressions of the tendency to disappear of the lateral xylems, whose efficiency 
as cotyledonary conducting channels is similarly restricted whether they end 
blindly above or below. 
We have seen that the change from the stable to the unstable type of 
tetrarchy is bound up with reduction in size of the seedling and the 
consequent restriction of the cotyledonary vascular system. It follows that 
to this prime cause we must also attribute the production of diarchy. 
With diarchy we once more enter upon a stable condition of vascular 
structure. While there is no doubt that diarchy bears a close phylogenetic 
relationship to the unstable forms of triarchy and tetrarchy, yet it is 
noticeable that it appears to be less variable than these other types of 
symmetry. Further, in the Genisteæ diarchy seems to be absolutely 
constant, no vestige of intercotyledonary xylems ever appearing. ‘Transitions 
to the Geniste-type from tetrarchy are afforded by Ononis spp. and the 
Podalyriew ; but within the tribe itself, despite notable variations in size 
and form, the symmetry is immoveably diarch. 
This rigidity of diarchy as compared with the plasticity of triarchy and 
tetrarchy in slender epigeal seedlings can be explained on these lines: the 
difference between triarchy and tetrarchy is often to be regarded simply 
as the expression of the alternate or opposite position of the first two leaves ; 
whereas between triarchy and tetrarchy on the one hand, and diarchy on the 
other, there is the more fundamental question whether or not the plumular 
traces shall be supplemented by special hypocotyledonary primary xylems at 
all. The further reduction of diarchy to monarchy seems to be almost out 
of the question in Dicotyledons. 
TRIARCHY. 
Having thus traced out a probable reduction series among the types of 
symmetry common in epigeal seedlings, we next enquire what is the phylo- 
* Excluding cases with more than four protoxylems, 
