158 de Frciine.—The Seedling Structure of certain Cactaceae. 
A. Species in which tubercle - and cotyledon-bundles play a part in 
root formation. 
Mamillaria multiceps , Salm-Dyck. The upper part of the hypocotyl 
contains four vascular bundles, two of which are supplied by the cotyledon 
and two by the epicotyledonary tubercles (Diagram 14, Fig. 1). 
Very soon four smaller epicotylar bundles make their appearance, 
alternating with the other four (Diagram 14, Fig. 2), and the eight bundles 
almost immediately fuse into two groups (e + t lt e + t 2 ), which take up 
Diagram 14. Mamillaria multiceps. 
a position in the intercotyledonary plane alternating with the two cotyle¬ 
don-bundles (c 19 c 2 ), which have by this time differentiated (Diagram 14, 
Fig- 3)- The two strands (c ls c 2 ) rotate slightly and pass inwards to meet 
the two rotating bundles e + t 1 and e-\-t 2 (Diagram 14, Fig. 4). The 
protoxylems of the adjacent bundles fuse, and by the continued movements 
of the phloems they are left in the exarch position (Diagram 14, Fig. 5). 
The opposite phloem groups finally fuse, resulting in the formation of 
a diarch root (Diagram 14, Figs. 5 and 6 ). In this species the metaxylem 
elements, which make their appearance towards the middle region of the 
