de Frame .— The Seedling Structure of certain Cactaceae. 163 
The other seedling of this species behaved as did the preceding one as 
regards the branching of the cotyledonary traces ; the branches, however, 
soon fused again, leaving only one cotyledon-bundle for each papillate seed- 
leaf. This bundle bifurcated at a lower level and the divided bundle played 
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Diagram 18. Mamillaria missouriensis. 
the same part in the transition as did the paired bundle of the first described 
seedling. In all other respects the two seedlings were precisely similar. 
The belated formation of a tetrarch root by the branching of the xylem 
arms recalls the similar feature described in Cereus tortuosus. 
C. Species in which cotyledons are absent , and epicotyledonary bundles 
alone produce the root structure. 
Mamillaria pusilla^ Sweet. The hypocotyl of these seedlings possesses 
in its upper region four vascular strands. These bundles move gradually 
towards the centre, and at the same time rotate slightly towards one another 
in pairs; finally the protoxylems of a pair fuse and metaxylem elements 
begin to appear centripetally, until a solid mass of large ‘ barrel ’ tracheides 
results. During this development the phloem-strands have been gradually 
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