3H 
Lee, — Observations on the 
element (Diagram 7, III). The stele is therefore perfectly symmetrical at 
this stage and in all the succeeding sections to the root-tip. Lower down, 
the metaxylem of the corner bundles becomes reduced to a single strand 
which passes inwards towards the centre, where new r lignified elements arise 
in such large numbers that the pith disappears. The groups of phloem 
and protoxylem having closed in also, a typical tetrarch stele results (Dia- 
gram 7, iv). 
In these seedlings there is a good deal of variation in the level at which 
transition first begins. In the midrib, for example, in some cases, the phloem 
bifurcates and the xylem divides into three before entering the hypocotyl. 
Seedling with three cotyledons . In this case one of the cotyledons was 
much larger than the other two. In the upper part of the hypocotyl the 
strands from each cotyledon behave as in the ordinary seedlings ; conse- 
quently, for some distance there are six protoxylem groups, and six other 
masses each consisting of xylem and phloem. Finally, the protoxylem and 
the metaxylem bundles from the smallest cotyledon die out, and rearrange- 
ments occur which result in the production of a tetrarch root. 
Tridax procumbens , Linn. Seedlings small (generally less than 4 cm. 
in length), each possessing two unequal cotyledons which unite at the base 
to form a very short cotyledonary tube. 
In the sessile cotyledons the vascular bundles are of the type, and 
follow the course, characteristic of the Compositae. A noteworthy feature 
is the early preparation for transition in the median bundle. As soon as 
the numerous vascular strands of the upper part of the cotyledon have 
given place, by fusion, to the usual midrib and two lateral bundles, the 
former bifurcates (Diagram 8, i). The two xylem portions thus produced 
remain in continuity by means of the protoxylem (which moves slightly to 
take up an exarch position), but the phloem halves diverge and become 
widely separated. Lower down, in the short cotyledonary tube, the lateral 
strands fuse in pairs, and the resulting bundles pass inwards nearly to the 
centre, where for a time they rejnain symmetrically disposed in the inter- 
cotyledonary plane. In the upper part of the hypocotyl, these bundles 
move round in an anti-clockwise direction (as seen from above), and finally 
come into contact with the two branches of the bifurcated midrib bundles 
situated at diametrically opposite corners of the square stele. This is the 
condition characteristic of the hypocotyl (Diagram 8, II). 
Near the base of the hypocotyl the symmetry is restored by the 
disappearance of part of the original intercotyledonary strands, and the 
complete fusion of the remainder with the corner bundles (Diagram 8, III). 
In the latter, at this level, no protoxylem can be distinguished. In the root 
most of the pith becomes replaced by vessels, and in the almost solid xylem 
core there are occasionally four narrow angles simulating a tetrarch structure. 
Finally, the xylem narrows considerably in the intercotyledonary plane, the 
