100 MR. R. H. COMPTON : AN INVESTIGATION OF THE 
this character is found to be unsound, we are justified in deciding against 
the view expressed by Tansley & Thomas and quoted above. 
It must be noted, however, that in one or. two instances in the Leguminosi 
cenogenetic explanations apparently fail, and the only alternative is an 
appeal to heredity. This is the case in Lotus Tetragonolobus, and may perhaps 
also apply to Trigonella cretica and Desmodium canadense (pp. 81, 82, 85). 
But apart from these instances, there is little support for a view which 
depends on the hereditary fixity of the level of transition. 
THE Type or SYMMETRY. 
The problem as to what determines the number of protoxylem groups in 
the hypocotyledonary axis is beset with difficulties, and a general solution 
cannot be expected in the present position of our knowledge. In the- 
Leguminosæ, however, some light can be thrown upon the subject, as certain 
correlations are found to exist between the type of symmetry and other 
characters of the seedling. 
In the first place, it appears that the characteristic type of symmetry in 
the Leguminose is the tetrarch. Among the 203 entries in column VII. of 
the Summary List tetrarchy is mentioned in 123 cases, as occurring at least 
in some specimens or in some part of the hypocotyledonary axis. Of the: 
other 80 entries, a great number are simply records of diarchy or triarchy 
in the root, and a further investigation would probably reveal a tetrarch 
phase in many of these cases also ; the Genisteæ, however, are typically 
diarch and the Vicieæ typically triarch (see pp. 79, 85). In particular 
the Cresalpinioideee and Mimosoidew are characteristically tetrarch, there 
being no well-ascertained example in which tetrarchy has not been found. 
It may be remarked that as the great majority of the trees are members of 
these two sub-orders, there also appears to be a correlation between the. 
tree-habit and tetrarchy, the significance of which will be considered below 
(p. 115). 
Again, it is evident from a study of the Summary List that in some- 
alliances the type of symmetry is remarkably constant: this is the case in 
the Mimosoideze, Cæsalpinioideæ, and Phaseolezx, where tetrarchy is the rule, 
Genisteze, which are uniformly diarch, and the hypogeal climbing Vicieæ, 
which have a peculiar form of triarchy. In other groups, on the contrary, 
there is great variability of type, this applying to whole tribes, genera, 
species, individuals, and parts of the same individual ; the Trifoliese, Lotez, 
a variability 
Galegez, and Hedysarex show this variability very strikingly 
that seems to circle round the unstable type of triarchy found in epigeal 
seedlings. 
Clearly, then, tetrarchy, characteristic of the Leguminosz, exists in two. 
forms, stable and unstable. What is the basis of this difference ? 
