de Fraine .— The Seedling Structure of certain Cactaceae. 133 
in position and forms one of the cotyledonary root-poles. These movements 
of the wood are somewhat masked by the rapid increase in the number 
of tracheides which arise in the centre of the axis; they do not long persist 
and have all disappeared by the time the tetrarch root has become 
established. 
In another example of this same species a modification of this process 
took place. The cotyledonary bundles were of the normal V-shaped type, 
part of their protoxylem became exarch by rotation, while the rest passed 
towards the centre of the hypocotyl and gave rise to a diarch plate; 
a little later this branched right and left and completed the tetrarch root- 
structure. 
Opuntia Bergeriana , Weber. The transition-phenomena of the single 
seedling of this species which was examined was of the type found in 
O. Ficus-indica. Epicotyledonary strands were present in the upper part 
of the hypocotyl; they were well differentiated, and at a lower level fused 
laterally with the cotyledonary bundles. Cambium was developed in the 
bundles and contributed a few secondary elements to the metaxylem, and 
in the tetrarch root secondary thickening had well begun. 
Opuntia strict a, Haw. 
Series A is in all essential features like O. Bergeriana. 
Series B is in the first stages of the transition identical with Series A, 
but the lateral xylem branches gradually die out and the hypocotyl in its 
middle region possesses only two ‘ double ’ bundles. In these features the 
seedling strongly resembles Pereskia (Diagram 1, Fig. 5); further, as in this 
species, a tracheid arising in the intercotyledonary plane and reinforced 
by others developed centripetally, once more restores the tetrarch root- 
structure. 
Series C is of interest in that the seedling possessed three cotyledons, 
each of which contributed one V-shaped bundle to the hypocotyl. Six 
epicotylar strands were also present, arranged in pairs between the 1 double 5 
bundles, with the nearest one of which they almost immediately fused. 
It was impossible to say which elements represented the protoxylem, for 
though a considerable quantity of wood was present the elements were 
practically indistinguishable from one another. Soon after the fusion of 
the epicotylar strands with the seed-leaf-traces, the xylem of the bundles 
separated into three parts, so that at this level there were in the hypocotyl 
six collateral strands of wood and bast and three isolated xylem groups. 
Towards the base of the hypocotyl the adjacent collateral strands rotated 
their xylem and fusion took place, new tracheides connected up these xylem 
groups with the three isolated strands of wood, and so a ring of xylem, 
with six protoxylems alternating with the six phloem bundles, resulted. 
‘ Barrel ’ tracheides of the type which will be described later under the genus 
Echinopsis , gradually developed in the centre of the axis until a solid 
