Seedling Anatomy of Certain Sympetalae. II. 323 
to the centre of the hypocotyl, travel obliquely and fuse with the midrib 
bundles. By further rearrangement a solid diarch xylem-plate is formed. 
Hieracium alpinum , Linn. Seedlings 7-9 cm. long, with comparatively 
large, more or less lanceolate seed-leaves which join the hypocotyl without 
forming a cotyledonary tube. 
The numerous vascular strands which occupy the upper part of the 
cotyledon give place at the base to three, the two lateral bundles being very 
small (Diagram 11, i). The rearrangement in the midrib strand begins at 
the base of the cotyledon, but the protoxylem does not become really 
exarch until the root is reached. In the upper part of the hypocotyl the 
corresponding laterals fuse in pairs, the resulting bundles passing inwards 
towards the centre (Diagram 1 1, II and III). During its passage, the proto- 
xylem becomes exarch, and the metaxylem gradually disappears. At 
a lower level there is a bisymmetrical tetrarch arrangement, the protoxylem 
groups being devoid of metaxylem, and the phloem being disposed in two 
flattened masses in the intercotyledonary plane (Diagram 11, III). Finally, 
the protoxylem elements enlarge, merge into the metaxylem at the centre, 
and give rise to a diarch root (Diagram 11, IV). 
In other seedlings examined the same sequence was found, but owing 
to the extreme smallness of the lateral bundles, the features were not 
exhibited with the same diagrammatic clearness in all cases. 
Theoretical Considerations. 
In a former paper on the seedling structure of the Tubiflorae (7) certain 
theoretical conclusions were put forward, to one of which allusion has already 
been made (vol. xxvi, p. 742). It was also shown that in the matter of 
classification no assistance is to be derived from seedling anatomy. In the 
Tubiflorae, genera, which on other grounds are held to be nearly related, 
differ markedly in the vascular anatomy of their seedlings, e. g. Incarvillea 
and Eccremocarpus (N. O. Bignoniaceae). 
The present case is rather different. In the fairly large number of 
seedlings examined, it cannot with reason be held that more than one type 
of transition has been discovered : for, although there are two extremes — 
diarch and tetrarch — all stages from the one to the other have been 
described in the preceding pages. The diarch type seen in Cichorium 
Intybus and Baeria coronaria passes by way of Solidago ulmifolia , Charieis 
heterophylla , and others (in which the laterals persist for a longer or a shorter 
distance without giving more than the slightest indication of tetrarchy) to 
Hieracium alpinum , Heliopsis laevis , and others (in which the laterals persist 
to form the weak intercotyledonary poles of a tetrarch root) ; and thence 
to such plants as Silphium perfoliatum (in which a fully symmetrical, tetrarch 
root is invariably present). That these are essentially stages of the same 
