724 Thomas . — Seedling Anatomy of 
are continuous either with the flanks of the double bundle (e. g. P. Malus, 
P. Ancuparia , and some individuals of P. communis ), as in Pi. LI, Fig. 25 
of Althaea rosea , or with independent laterals (e. g. A and some 
individuals of P. communis ), as in PI. L, Fig. 10 of Magnolia tripetala . 
The forms which have four groups of protoxylem in the hypocotyl 
possess a tetrarch root, e. g. P. Aria , while those which show six proto- 
xylem groups in the hypocotyl may, or may not, reduce lower down. 
In one individual of P. communis both the intercotyledonary protoxylem 
group and the diagonal groups were present, and the root was heptarch, 
obviously through suppression of one diagonal pole of an octarch condition. 
It is noteworthy that these differences bear no obvious relation to habit 
of seedling, but would appear to depend upon some inner cause as to the 
nature of which one can, in the present state of knowledge, only speculate. 
Pyrus communis would seem to be by far the most variable species, but 
fewer individuals of the other forms have been examined. 
The difference between tetrarchy and hexarchy is of the same order as 
the difference between fused and free laterals in the base of the cotyledon, 
since in the first case the lateral strands unite to form one pole of the root, 
as in P. Aria and some individuals of P. communis (cf. PI. LI, Fig. 26 of 
Althaea rosed) , or remain apart, as in other individuals of P. communis. 
This union or separation in the root, however, is not correlated with the 
union or separation of the strands in the base of the cotyledons, since all 
combinations occur, as will be seen by reference to examples and diagrams 
(Text-fig. 41). 
Cotoneaster affinis. The hypocotyl shows the four groups of phloem 
with more or less metaxylem within, according to age. In the cotyledonary 
plane are the protoxylem strands. The usual triad passes out to the 
cotyledons, constituting the double bundle which at the node differentiates 
lateral strands on the flanks, that in some cases do not separate from the 
central bundle until some distance up the cotyledons. The diarch root is 
produced in a somewhat leisurely manner. 
Crataegus Oxyacantha is very similar to Cotoneaster. 
Saxifragaceae. 
Saxifraga muscoides , .S'. Aizoon , Heuchera alba, Philadelphus grandi- 
floras. These are very slender seedlings with a very high transition to 
a diarch root. 
Crassulaceae. 
Sedum glaucum is a very swollen seedling with high transition to 
a diarch root. 
Cotyledon orbicidare is another very globose form with extremely 
minute vascular strands which form a diarch root just below the node. 
