Seedling Structure of certain Cen trospermae. 177 
the protoxylem is brought into the exarch position but still remains in the 
plane of the cotyledons; finally, the two groups of phloem- and of metaxylem- 
elements of one cotyledonary bundle fuse with the corresponding tissues 
of the other cotyledon, the resulting strands being situated in the inter- 
cotyledonary plane. A diarch root-structure is thus formed. 
This is Van Tieghem’s third type of transition, and it will be referred 
to in the following pages as type 3. 
Calandrinia speciosa shows no important feature of difference from 
C. Menziesii. 
Calandrinia amoena , Vis., in all essentials is identical with C. Menziesii ; 
the cotyledons, however, are not so rounded and the bifurcation of their 
central bundles does not take place at so high a level 
Calandrinia grandiflor a, Lindl, is chiefly remarkable from the fact that 
the rearrangements of the cotyledonary strands are more complete at a higher 
level than in any of the other species of the genus examined, the rotation of 
the bifurcated bundle being carried so far that the two groups of metaxylem 
elements are closed up, with the protoxylem in the exarch position, at 
a level just above the cotylar tube (Diagram 1, Fig. 7). This feature is 
also shared by C. umbellata , although not in so marked a degree. 
Claytonia perfoliata, Don. The main features of dissimilarity between 
the transition phenomena in the very small and slender seedlings of this 
plant and those of species of Calandrinia are that the two lateral bundles 
of each seed-leaf of Claytonia early fuse with the larger central one ; the 
main cotylar bundles are endarch and collateral through the whole length 
of the seed-leaf ; and, finally, the vascular rearrangements which take place 
in a very short vertical distance leading to the diarch root-structure occur 
during the passage of the seed-leaf-traces from the cotyledons to the central 
region of the axis and in the upper part of the hypocotyl. 
Portulaca oleracea ) L. The seedling is very small and its cotyledons 
are much flatter than those of any of the preceding plants. The seed-leaves 
have several bundles in their blades, one large central one with two or three 
smaller strands on each side. The latter fuse on to the main central bundle, 
which shows bifurcation and rotation before the last of the laterals have 
effected a junction with it in the basal region of the blade (Diagram 1, 
Fig. 8). The transition is of type 3 and closely resembles that of Calan- 
drinia umbellata. 
CARYOPHYLLACEAE. 
SILENOIDEAE. 
Cucubalus baccifera, L. The transition resembles that of the foregoing 
plants in following type 3. Of the differences, relatively unimportant, which 
obtain between Cucubalus and Calandrinia for example, the following are 
