Anatomy of Teratological Seedlings. IV. 467 
this, although where the protoxylem is lost high up in the hypocotyl no 
corresponding lateral root or root-pole is usually formed. The formation of 
one additional root-pole in this manner is a fairly common feature of both 
types of seedling, and results in either a triarch or a tetrarch condition. It 
is due in such cases to the asymmetrical persistence of one lateral only. 
In the previous paper ( 18 ) dealing with these seedlings a comparison of the 
independent laterals with the ‘Zwischenstrange’, first described by Dodel ( 11 ) 
and later by Compton ( 5 , 6), was made and the differences between the two 
types noted. This difference has received emphasis by the discovery of 
strands showing much closer affinities with true ‘Zwischenstrange’ in the 
series of seedlings now described. These strands, each of which normally 
gives rise to a new member of the lateral root-whorl and subsequently 
to a root-pole, usually originate in the basal third of the hypocotyl, and at 
first are represented by an isolated tracheide (Fig. 29) lying in the plane at 
right angles to the sagittal plane of the seedling. This is augmented at 
a lower level by metaxylem, and ultimately forms what to all appearances is 
a normal exarch hypocotyledonary xylem group. In some seedlings the 
new protoxylems arise at a much lower level, and may be evident only at or 
near the level at which the whorl of lateral roots is produced. Exceptionally 
they may not be represented in the hypocotyl at all (Figs. 36, 37, 41, 42), 
and then are evident only as new root- poles. The fact that a series of 
stages is present leading from one extreme to the other leaves little doubt, 
however, as to their being homologous. 
The production of a single additional root-pole by this method is the 
one most commonly occurring in typical syncotylous (‘ Type 1 ’) seedlings, 
whilst the development of two such poles, one on either flank of the cotyle- 
donary midrib, is very common in ‘ Type 2 ’ seedlings, tetrarchy resulting in 
both cases. The asymmetrical development of one of these accessory 
strands alone in ‘Type 2’ seedlings is rare and results in a triarch 
condition, but it is interesting to note that tetrarchy may be produced by 
a combination of this type with an independent lateral on the opposite side 
and is indeed fairly common (Figs. 30-34). Only a single ‘Type 1 ’ seed- 
ling (Figs. 35-38) has been observed in which this combination of the two 
methods of additional xylem pole formation has occurred. Of the two 
poles, the one derived from the independent and persistent lateral alone 
gives rise to a lateral root (Fig. 37 c), whilst the second pole (Fig. 38 e) is 
only differentiated in the tap-root. There are thus four lateral roots in the 
root-whorl, whilst the tap-root is pentarch. The production of two new 
poles in ‘Type 1 ’ seedlings is extremely rare and has only been met with 
in two instances. One of these (Seedling A), in which the syncotyly is ex- 
tremely intimate, has already been described. The second (Fig. 5) is one 
( in which the additional poles only arise in the tap-root, which is thus 
pentarch, although the root-whorl consists of three members only (Figs. 39- 
