5&2 Dcivey . — Seedling A natomy of certahi Amentiferae. 
becomes more compact at lower levels, while lignification proceeds centri- 
petally rather than tangentially from the protoxylem centres. In the 
region of the external collet the walls of the endodermis become much 
thickened. Tetrarchy persists throughout the root. 
Myrica Gale . The structure of the cotyledonary traces and their 
behaviour on entering the axis resembles what is found in M. californica. 
At the node the cotyledonary and plumular strands unite to form a very 
compact stele in which the pith soon becomes obliterated by lignified 
elements. The cotyledonary phloems unite in the intercotyledonary plane 
in the manner of the diarch type. At a high level the stele shows 
the structure of a diarch root with a somewhat broad xylem plate, sur- 
rounded by a well-differentiated endodermis (Fig. 4). Diarchy extends 
Figs. 4 and 5. Myrica Gale. 4. Transverse section of lower part of hypocotyl, showing 
diarch root- structure. 5. Transverse section of root of same seedling near its apex. cot. px. — 
protoxylem of cotyledon ; ph. = phloem. Both x 480. 
throughout the hypocotyl. In the region of the collet extensions of the 
xylem mass in the form of metaxylem and exarch protoxylem elements 
bisect each phloem group in the intercotyledonary plane. The cruciform 
tetrareh arrangement thus produced is continued downwards throughout 
the root (Fig. 5). 
The above species agree in the possession of a tetrareh root, and also 
in the diarch structure of their hypocotyls near the cotyledonary node. In 
M. Gale the intercotyledonary protoxylem poles do not extend upwards 
above the collet, and the hypocotyl structure is that of a diarch root, 
but in M. californica four phloem groups always remain distinct, and the 
intercotyledonary protoxylem poles extend upwards through the greater 
part of the hypocotyl. 
