132 de Frame.—The Seedling S true hire of certain Cactaceae. 
towards the centre of the axis, where it fused with the similar protoxylem 
group from the other cotyledonary bundle ; metaxylem elements rapidly 
formed on either side of the plate thus produced. At a lower level one 
half of this intercotyledonary protoxylem plate died out, the other half 
moved rapidly outwards and formed the third root-pole (Fig. 5, px^ +px 2 ') 
and soon became connected by metaxylem elements with the other two 
protoxylems. The cotyledonary xylem groups (Fig. 5, px Y and px{) 
again branched (px 2 and px 2 ) and the fourth protoxylem of the tetrarch 
root was established. 
Fig. 5. Opuntia Tuna. Series C. x 220. 
Two epicotyledonary strands were well developed in the upper part 
of this seedling and, during the early stages of the transition, they appeared 
to be about to take part in the root formation ; they did not do so, however, 
and finally gradually died out. 
Opuntia poly ant ha, Haw. Each cotyledon supplies the hypocotyl with 
one £ double ’ bundle, in which the xylem is arranged in three groups ; very 
few xylem elements are present and in all probability no metaxylem has 
as yet developed. The lateral xylem groups of the £ double ’ bundle pass 
outwards towards the intercotyledonary plane and there fuse with the 
similar groups from the other ‘ double ’ bundle ; the median group remains 
