592 
Holden and Dexon.—On the Seedling 
Of the three types we believe the last to be the most recently evolved, 
and to be the one which gives the clue to the origin of the majority of those 
forms showing diagonal tetrarchy in the root (e. g. Calycanthus ). 
We consider that the sycamore exhibits this lateral concentration and 
that there has been a definite moving outwards of the major portion of the 
midrib constituents. It represents a relatively early stage, however, of this line 
of evolution, and the vascular components forming the compound diagonal 
bundle are soon dissociated, probably in part at least owing to the importance 
of the laterals which form the intercotyledonary poles. We may trace the later 
stages in a number of seedlings. In Lupinus hirsutus} L. albus , and L. muta- 
bilis for example, the diagonal arrangement extends throughout the hypocotyl, 
and a still later stage is exhibited by the seedlings of Abronia spp. ( 9 ). Here 
the diagonal grouping is not only continued through the whole of the hypo¬ 
cotyl, but involves the upper part of the root. The result is that well- 
developed root poles are developed in the diagonal position, whilst only 
a rudimentary pole, represented by the isolated protoxylem, is present in 
the cotyledonary plane. This condition may be persistent, but more usually 
there is a gradual return to the older symmetry and the diagonally situated 
poles dwindle whilst the cotyledonary poles become progressively more 
important, so that the tap-root is ultimately diarch. The situation is com¬ 
plicated somewhat in Abronia owing to the fact that the lateral concentration 
only affects one cotyledon, the other, which is much reduced in size, showing 
median concentration (Fig. 73 a). The final term in this series is shown by 
forms like Calycanthus. Here the diagonal position is permanent throughout 
the seedling, so that four diagonal root poles occur without normally any 
reversion to an ancestral diarchy. It is interesting to note, however, that 
Thomas ( 16 ) records a reversion to the diarch state as an occasional 
abnormality in Calycanthus. A further point of similarity in the series is 
also worthy of mention. There exist in Calycanilms a number of small 
secondary collateral bundles at the base of the cotyledons between the 
widely separated diagonal strands, these appearing quite soon after germina¬ 
tion has commenced. A single strand of a similar character is frequent in 
Lupinus mutabilis , though here it appears later, and it is also present as a 
rare abnormality in the sycamore. There is thus good reason for regarding 
the series Acer-Lupinus-Abronia-Calycanthus as illustrating a well-marked 
tendency in seedling anatomy. 
It is not proposed to discuss in this paper the possible significance of 
the vascular structure of the leaves and epicotyl, as it is felt that more work 
of a comparative character is necessary before its real bearing can be 
adequately determined. 
1 Thomas ( 16 ) records a number of species showing a condition similar to that found in 
Lupinus hirsutus f e. g. Decaisnea Fargesii, Laurus Sassafras, Liquidambar styraciflua. 
