de Fraine .— The Seedling Structure of certain Cactaceae. 161 
movement of the tubercle and epicotyledonary strands until they occupied 
a position similar to that indicated in Diagram 16, Fig. i, and by the 
subsequent fusion of the three bundles lying between the cotyledon-traces, 
the eight bundles originally present were reduced to four, which performed 
during the remainder of the transition in the manner indicated for 
M. multiceps in Diagram 14, Figs. 3-6. 
The seedling in which no tubercle-bundles appeared showed a transition 
very closely resembling that described for M . rhodantha , but the four 
bundles occupying the intercotyledonary plane in the latter species and 
shown in Diagram 15, Fig. 3, were, in Mpentacantha.zM epicotylar in origin. 
Mamillaria eriacantha , Hort. As in M. pentacantha, a tubercle-bundle 
supplying the cotyledonary bud may or may not be present; but, unlike that 
species, the transition takes place in essentially the same manner in both 
cases. Two cotyledonary and two epicotyledonary bundles are present at 
the top of the hypocotyl ; when the tubercle-bundles are developed they 
simply move outwards and fuse with the epicotyledonary strands (Diagram 
17, Figs. 1 and 2). Almost concurrently with this fusion the cotyledonary 
bundles bifurcate, their phloems separate and rotate until they lie on either 
side of the protoxylem, which by this means is left exposed ; during this 
change large metaxylem elements appear, in the position indicated in Diagram 
17, Fig. 3. The epicotyledonary strands have by this time moved laterally 
towards the cotyledon-traces, fusion with them takes place, and the resulting 
bundles give rise to a diarch root in a precisely similar way to that 
indicated in Diagram 14, Figs. 5 and 6. 
B. Species in which only cotyledon-bundles play a part in root formation. 
(Strictly speaking some seedlings of M. pentacantha and M. eriacantha 
fall into this category and not into Group A, but for the sake of simplicity 
they have been described above.) 
M 
