EMBRYOGENESIS IN FLOWERING PLANTS 265 



on one side and convex on the other, thus appearing somewhat 

 kidney-shaped when observed in transverse section. The apical 

 concave end of the embryo becomes still further enlarged and gives 

 rise to the two lobes of the cotyledonary lamina, which at first are 

 folded together in such a manner that the tissues which sub- 

 quently become the upper surface of the two lobes are in contact 

 with one another. The cotyledonary petiole next elongates rapidly ; 

 the petiole is grooved on the upper portion at maturity but is only 

 slightly flattened or circular in transverse section throughout the 

 greater part of its length. A second concavity at the basal end of the 

 petiole represents the base of the original lateral concavity present 

 at a younger stage. Certain cells with large nuclei and prominent 

 nucleoli which early develop near the base of the lateral concavity 

 mark the position where the stem apex subsequently arises lateral 

 to the cotyledon and surrounded by its sheathing base. Opposite 

 the cotyledon there is developed a small hump of tissue, occupying 

 the position where the second cotyledon would have arisen if it 

 were actually present (see Metcalfe, 1936). Its position and mode of 

 origin indicate that it is the rudiment of a second cotyledon which 

 fails to develop. This hump, like the other cotyledon, is supplied 

 with a provascular strand. As a matter of fact, seedlings with two 

 cotyledons have been observed, hence the rudimentary cotyledon is 

 actually capable of normal development.' 



If the embryological evidence indicates that the single cotyledon in 

 monocotyledons is a truly terminal organ, explanations rather diff'erent 

 from those outlined above must be sought. Several investigators state 

 emphatically that the cotyledon does originate as a terminal structure, 

 the shoot apex being situated in a lateral position near the base of the 

 embryo. Taylor (1921) considers that explanations based on the 

 conception of unequal cotyledonary development, or of the early 

 abortion of one cotyledon, do not accord with the facts as ascertained 

 in Cyrtanthus parviflorus; and this appears to be true of the embryos 

 of Alisma (Hanstein, 1870), and Sagittaha (Schaffner, 1897). In the 

 Alismaceae, as observed by Johri (1935, 1936), the first division by a 

 transverse wall divides the zygote into a large basal and a small terminal 

 segment, as in Limnophyton obtusifoUum, Fig. 68. The terminal cell 

 gives rise to the whole of the embryo and also contributes to the upper 

 region of the filamentous suspensor. The basal cell, as in Capsella, 

 becomes greatly enlarged in Sagittaria, Limnophyton, Alisma and 

 Butomopsis. 



In Lilaea subulata (Juncaginaceae) the initial embryology shows no 

 special features (Agrawal, 1952). Later, however, the embryo consists 



