2 62 
Hill and de Fraine.—A Consideration of the 
of all the modifications found from a tetrarch than from a diarch form 
since diarchy is associated with extremely definite and constant features 
which suggest a certain ‘ stereotyped rigidity unfavourable to the evolution 
of new forms ' ; on the other hand, tetrarchy, particularly as seen in Gymno- 
sperms, c gives an impression of plasticity and variability ’ and ‘ ample scope 
for the derivation of all the modifications seen \ It must, however, be noted 
that the diarch rather than the tetrarch type of symmetry is characteristic 
of Gymnosperms — in fact, even triarchy is more usual than tetrarchy 
(Table I). Further, there is considerable reason for believing that it is the 
size of the seedlings which is the determining factor in producing the par- 
ticular type of symmetry ; but this is a question which will be considered at 
further length later. 
The existence of an intermediate type between diarchy and tetrarchy 
has been demonstrated in certain plants, e. g. Z iriodendron tulipifera , Clematis 
Hendersoni , and some Composites ; in these the lateral bundles of the cotyle- 
dons enter the hypocotyl and attempt to form the intercotyledonary poles 
of a tetrarch root. 
According to Miss Thomas, this is to be regarded as ‘an ancestral 
feature not completely eliminated ’, for she finds it ‘ difficult to see what 
functional advantage would accrue from such a very abortive attempt to 
form the intermediate poles of a tetrarch root’. This intermediate type 
appears to be a fairly widespread one so far as our observations go, and it 
seems to be confined to large, or moderately large, seedlings. In small 
seedlings the one lateral bundle, so generally present on either side of the 
midrib of the cotyledon, fuses with the main bundle at a varying distance 
above the cotyledonary node, hence two bundles (bifurcated or not) enter 
the node, and the transition to a diarch root is rapidly effected. As the 
seedlings increase in size, so the tendency for the delay in the fusion of the 
laterals with the main bundle increases, resulting usually in the production 
of a tetrarch structure in the hypocotyl ; this tendency finds its ultimate 
expression in the production of a tetrarch root. Size of seedlings almost, 
if not quite, determines whether or not the laterals shall penetrate the 
hypocotyl or fuse with the main cotyledonary strand before its entrance. 
The reason for the relation which exists between the delay in the fusion of 
the laterals and the size of the seedling is to be found, in all probability, in 
the physiological needs existent at this stage of growth ; the larger the 
seedling the greater the need for increased vascular supply. This finds 
its expression in the expansion of the diarch to the tetrarch type, 
or at any rate in the production of a more or less persistent tetrarch 
stage. Correlated with this is the presence of a stem-structure in the 
hypocotyl. 
If seedling-structure is to be used as an indicator of affinity, several 
difficulties are met with. 
