5 1 8 W or s dell . — The Morphology of the Monocoty ledonou s 
divisions of vascular plants. In this connexion I cannot do better than 
quote Celakovsky : 
‘ The cotyledon of Monocotyledons is, in my opinion, primitively 
single and terminal. The Monocotyledonous embryo is, before the bud and 
root are laid down, as Hanstein has stated, a simple thallus. This is com- 
pletely homologous and equivalent to the sporogonium of Bryophytes 
in the embryonic state of the latter, as is shown by the similar mode 
of cell-division. The further construction of the thallus is indeed different 
from that of the sporogonium, corresponding to the phylogenetic advance 
from the earliest stage of the Thallophytes to the second stage of the 
Vascular Plants.’ ‘ The simplest primitive metamorphosis, according to my 
repeatedly-expressed view, consisted in the development of the upper 
terminal portion of the embryo (which .in the Mosses became the spore- 
capsule) as a purely vegetative assimilating organ, viz. a leaf, as occurs, 
according to Kny, in Ceratopteris among the Ferns before branching of any 
kind occurs, and the same thing is repeated in Monocotyledons. Just 
as the Moss-sporogonium becomes differentiated into two parts — the basal, 
sterile seta, and the terminal spore-capsule — in the same way is differentiated 
the embryonic thallus of Monocotyledons into the terminal leaf (cotyledon) 
and the basal stem-segment (hypocotyl), so that thus the Moss-capsule 
is phylogenetically homologous to the cotyledon, and the seta, or at any 
rate its basal portion, to the hypocotyl. In the embryonic thallus of 
Ceratopteris and of Monocotyledons, including the Grasses, the stem-bud 
arises laterally. . . . The embryonic thallus must be held to be the first seg- 
ment (Glied) of the leafy shoot ; its hypocotyl represents at a later stage the 
first stem-segment of the further developed embryonic shoot, and its 
cotyledon the first leaf of the latter." 
The importance of this comparison of embryological structure in the 
different main plant-groups has never been adequately realized. The basis 
of comparison must be a perfectly sound one, as embryos are the least 
variable of all structures, and thus the most likely of all to reveal ancestral 
features. Hence, if the embryo of Monocotyledons and that of Ceratopteris 
exhibit the same construction as the sporogonium of Bryophytes, the con- 
clusions deduced therefrom as set forth above are perfectly legitimate. The 
fact that Bryophytes are so distantly separated, in the genealogical tree 
of the Vegetable Kingdom, from Pteridophytes and Angiosperms can make 
not the slightest difference, for the embryo-structures, with their unvarying 
mode of development, constitute intimate connecting links at every stage. 
The view above set forth could only be overthrown if it could be shown 
that the terminal position of the cotyledon in Monocotyledons and Cerato- 
pteris has arisen as a secondary modification of the condition obtaining 
in Dicotyledons 1 and in other Ferns. There is, however, no real 
1 The supposed two cotyledons of this class are, as Hegelmaier points out, present before there 
