Seedling Anatomy of Certain Sympetalae. II. 325 
Several writers have emphasized the fact that in general seedlings 
have either a diarch or a tetrarch root. The connexion between these two 
types has been shown above. Only in exceptional cases is some other 
arrangement found to hold, e. g. in the Liliaceae ( 10 ). If, in the general 
case, evolution of the seedling anatomy has not been an extremely slow 
process, it is quite possible that diarchy and tetrarchy have been gained and 
lost many times during the evolution of the Angiosperms. This may help 
to explain the presence of the two types of seedling structure in the Com- 
positae, which by universal consent is the most highly organized Natural 
Order in the Plant Kingdom. In this group, if there are cases, as 
Miss Thomas assumes, where the tetrarch structure is becoming reduced 
to diarch, there are as certainly seedlings in which the diarch structure is 
giving place to tetrarch. 
In the general description of the venation of the seed-leaves (p. 305) it 
was shown that there are always present more or less numerous laterals in the 
cotyledon. On a first consideration it appears that the conditions which 
regulate these laterals determine the type of root to be produced ; that is, 
that the size of the laterals and the level at which they unite with the 
bundles in the cotyledonary plane might well determine the type of root 
structure. But the problem is not so easily solved. Compared with the 
respective midrib bundles, the lateral strands in the species which possess 
fully symmetrical tetrarch roots are often no larger than the laterals which 
form the core of the diarch root in the diarch species ; while in such 
examples as Bidens pilosa and Zinnia pauciflor a , both of which are strongly 
tetrarch, they are much smaller and are more or less insignificant. 
The level at which the laterals unite with the midrib strand varies 
greatly. In the extreme cases It is true that in the diarch seedlings this 
union occurs in the cotyledon or in the upper part of the hypocotyl ; while 
in the extreme tetrarch seedlings the union occurs near the base of the 
hypocotyl. But between these two extremes there is every gradation, as 
will be seen from the descriptions in the preceding pages. To the present 
writer it seems obvious that there must have been some other factor at 
work in addition to the size of the laterals and the level at which these unite 
with the midrib bundles. 
According to Hill and de Frame (6), 5 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/ i. e. size of 
seedling determines whether a diarch or a tetrarch arrangement shall be 
produced. A similar conclusion was reached by Compton in studying the 
Leguminosae (1). In the Compositae, as well as in the Tubiflorae, it is 
generally true that large seedlings are tetrarch and small seedlings diarch, 
but there are many conspicuous exceptions. An inspection of the outline- 
drawings given in Text-fig. 1 will save discussion. The drawings are all 
z 
