486 Holden and Daniels . — Observations on the 
sheath of varying length. If both growing points continue to develop 
equally the dicotyledonous condition is attained. If one of the growing 
points ceases to develop, the growth of the whole cotyledonary zone is 
associated with that of the other growing point and the monocotyledonous 
condition is reached. Monocotyledony is thus due 'neither to syncotyly 
nor to cotylar suppression, but is simply the result of the continued develop- 
ment of one growing point of the cotyledonary ring rather than a division 
of the growth between two growing points. Cotyledons are thus invariably 
lateral structures. The variation in the cotyledon number which has led 
to the separation of the two great Angiosperm series is considered to have 
arisen, in all probability, in those forms which possessed a massive pro- 
embryo of a type similar to that of the Liliales, Arales, and many Ranales 
among the modern groups. The massive proembryo is therefore held to 
represent the primitive proembryonic condition, the filamentous type of 
Alisma and Capsella being derived from this. 
The evidence marshalled in support of this theory lies chiefly in the 
interpretation placed upon the intermediate stages of embryonic develop- 
ment in certain Monocotyledons, notably Agapanthus umbellatus and 
Cyrtanthus sanguineus , but also on the comparative morphology of mono- 
cotyledonous and dicotyledonous seedlings of the former. The fact that 
Agapanthus produces, though apparently rarely, dicotyls as well as 
monocotyls is considered to be significant, since, it is argued, if this is the 
case there must be some evident relationship between the two conditions. 
The vascular system of the normal seedlings of Agapanthus is simple. 
The cotyledonary vascular supply consists of a median bundle with a lateral 
on either flank. These unite at the node, where they are joined by the 
midrib of the first plumular leaf. The strand from the latter forms a root- 
pole, whilst the cotyledonary strands between them form two poles so that 
a triarch root is organized. At a lower level a tetrarch condition arises 
owing to the bifurcation of the pole derived from the first leaf-strand. 
From the fact that the primordia of the cotyledon and first plumular leaf 
are developed first and the vascular strands subsequently laid down in 
these, it is inferred that the vascular features are determined by the pri- 
mordia rather than the reverse. The vascular strands, in fact, are held to be 
‘ secondary structures whose appearance is dependent upon the character of 
the primary structures and therefore of no phylogenetic significance’. 
The dicotylous specimen described shows two epicotyledonary leaves 
also. Each cotyledon is supplied with two strands in lateral positions 
‘ as if the middle one present in the monocotyledonous seedling has not 
been laid down ’. Each epicotyledonary leaf has a single median strand. 
Six strands thus unite at the cotyledonary node, and from these, as in the 
monocotylous seedlings, three root-poles are organized, an additional pole 
being developed by the bifurcation of that continuous with the midrib of 
