Davey.— Seedling Anatomy of certain A mentiferae. 593 
may be incurved, the laterals occasionally forming almost complete rings. 
The cotyledonary traces enter the hypocotyl and are continued downwards 
as triad units in accordance with the hexarch type described for Quercus . 
At the cotyledonary node, plumular leaf-traces in the form of slightly 
differentiated bundles or desmogen strands are interpolated between the 
Figs. 16 and 17. Castanea sativa. Details of vascular units indicated in Fig. ig. 16. A 
‘plumular’ unit. 17. A cotyledonary unit (only one of the constituent phloem groups is shown at 
ph.). px. = protoxylem ; mx. = metaxylem. Both x 360. 
cotyledon traces (Fig. 14). In descending the hypocotyl there is increase 
in the differentiation of the xylem elements of any pair of adjacent plumular 
strands, while an isolated protoxylem group appears between them. The 
vascular system of the hypocotyl thus comes to consist of a variable number 
of triad units, of which only six are related to the cotyledon traces (Fig. 14). 
The number of additional units may vary from two to eight, but is commonly 
six. At lower levels the lateral or tangential metaxylem of each triad fails 
