576 Holden and Bexon .— On the Seedling 
of three, one associated with each of the two leaves. In older seedlings 
(root (Fig* 14) the structure of the hypocotyl is essentially the same, 
but it is noteworthy that secondary thickening is very evident in the coty¬ 
ledonary midrib (Fig. 15) and is beginning at the upper end of the hypocotyl 
(Fig. 17) whilst the protoxylem is already disorganizing. The transition to 
root structure takes place below the collet and proceeds very slowly with the 
ultimate establishment of tetrarchy (Figs. n-13). During the assumption 
of root structure the metaxylem groups flanking the cotyledonary plane 
gradually approach the protoxylems in that plane and fuse with them, thus 
constituting two root poles (Figs. 19-21). This method is constant in all 
seedlings. The behaviour of the strands concerned in the formation of the 
root poles in the intercotyledonary plane is, however, somewhat variable. 
In some seedlings a central protoxylem group appears in the intercotyle¬ 
donary plane, the adjacent metaxylems fusing with this to form a root pole. 
In other cases there is no such differentiation of a central isolated protoxylem, 
but the two metaxylems simply approach one another and after fusion 
become organized into a normal root pole. The approximation of the 
metaxylem groups may be accomplished by the tangential development of 
elements in continuity with the main body of elements, or in many cases 
elements may appear irregularly on the inner side of each metaxylem group, 
first in isolation, then linked up with the main group. 
The tetrarch root in all cases possesses a large pith throughout its 
whole length. 
It is interesting to note that from the time that the cotyledons are un¬ 
rolled the bulk of the cotyledonary xylem (Fig. 22) and that associated 
with it in the hypocotyl are secondary and the primary elements occupy 
a subordinate position in the conducting system. The six plumular strands 
connected with the two epicotyledonary leaves show little differentia¬ 
tion at this stage. The median strand consists of a fairly well developed 
phloem group, whilst the development of the xylem is just beginning, a single 
element appearing within the phloem. The two lateral strands, which are 
small and undifferentiated, move outwards to join the adjacent diagonal 
phloem groups. The behaviour of the median strand is very variable. In 
some cases it bifurcates, each half moving across as a small collateral strand 
to join the adjacent lateral strand, whilst in others the phloem alone divides 
and the xylem remains central to the lowest level of differentiation. In 
other cases, again, the bundle does not divide at all and moves as a unit to 
the right or left. Even in the same seedling the behaviour is inconstant, 
one midrib behaving in one way and one in another. The plumular xylem 
does not appear to have any influence whatever on the formation of the 
intercotyledonary root poles, since many seedlings occur in which the plumu¬ 
lar midrib forks at the cotyledonary node or moves laterally, whilst a new 
intercotyledonary root protoxylem is organized below the collet. 
