138 de Fraine .— The Seedling Structure of certain Cactaceae. 
cells, practically without intercellular spaces. Towards its base downward 
growth may occur in two or more regions, resulting in a curious cortical 
lobing of the upper part of the slender root ; a fuller description of this 
phenomenon will be given later. In other cases the passage from hypocotyl 
to root is only accompanied by a great diminution in the size of the cortical 
cells. The short primary root usually possesses a slight development 
of cork in its outer cortical region. 
The transition from stem- to root-structure is remarkably uniform 
throughout the genus ; the phenomena will therefore be described for one 
species, and only the slight and usually unimportant differences exhibited 
by those other species which have been examined will be noted. 
Transition. 
Cereus tortuosus , Forbes. Each cotyledon of the seedlings of this 
species has one small endarch bundle throughout its length, but towards 
the base very small lateral strands appear, one on each side of the median 
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Diagram 5. Cereus tortuosus. 
one, in which, by this time, the phloem has bifurcated. The lateral 
bundles run obliquely through the mesophyll and fuse with the median 
strand. About this level a fourth bundle makes its appearance ; it originates 
in the cotyledonary bud region and evidently provides its vascular supply. 
This cotyledonary bud bundle moves rapidly outwards, rotating as it does 
so, and finally fuses laterally with one portion of the cotyledonary bundle 
(t v 4, Diagram 5, Fig. 1). 
During these movements the bundles have entered the hypocotyl, where 
four small epicotylar strands ( e ) may or may not be present; if they are 
differentiated they pass outwards and fuse with the seed-leaf-traces (Diagram 
5, Fig. 2). With the fusion of the cotyledonary bud-bundle with the 
seed-leaf-strand the transition is practically effected, for in general no further 
