152 EMBRYOGENESIS IN PLANTS 



cell of the shoot can be distinguished near the base of the leaf and 

 away from the root initial, its position being clearly indicated sub- 

 sequently by the cluster of associated hairs. (5) In some embryos it 

 appeared that the shoot, leaf and root may all have originated from 

 the epibasal segment, the hypobasal segment giving rise to the foot 

 only; this point, however, was not definitely ascertained. The root is 

 apparently endogenous in origin and in alignment with the leaf; it may 

 sometimes emerge in proximity to the archegonium neck. (6) As in 

 some other ferns, the shoot is slow in developing. Its associated stele 

 is formed later than that which runs between the leaf and the root; 

 it becomes conjoined with this strand. (7) Many embryos may develop 

 on a gametophyte at the same time. In such cases it was observed that 

 starch may accumulate round the half-formed embryo. (8) The foot is 

 typically a large organ but in older embryos it tends to be in contact 

 with the cushion only at its base. As a resuh, the embryo projects 

 strongly. 



If we assume that the rather tenuous parental prothallial tissue has 

 relatively little regulative effect on the zygote, and also bear in mind 

 that archegonia may be disposed on the prothallus with the neck 

 pointing upward, downward or laterally (because of curvatures in the 

 cushions), then the variable position of the first partition wall of the 

 zygote is perhaps to be expected. It seems to be fairly clear that, as in 

 other archegoniate plants, the polarity of the embryo is defined once 

 this wall has been formed. From this beginning, the further organogenic 

 development of the embryo is not unlike that in other leptosporangiate 

 ferns ; it is reminiscent of some eusporangiate ferns in the delay in the 

 inception of the primary organs. Some of Holloway's observations 

 suggest that in the nutrition of the young embryo there may be a 

 preponderance of carbohydrates relative to the other substances re- 

 quired for sustained growth. This might account for the late inception 

 of the leaf, root and shoot apices. In the illustrations in Fig. 35 bis, 

 the present writer has given the embryos the orientation which he 

 thinks they may have had during their actual growth. 



The Matoniaceae occupy a position related to, but somewhat 

 in advance of, the Marattiaceae and Gleicheniaceae (Bower, 1926). 

 Stokey and Atkinson (1952) have cultured the spores and obtained 

 prothalli, embryos, and eventually young plants of Matonia pectinata. 

 Fig. 33m. During its development from the spore the gametophyte 

 passes through a typical filamentous stage, organises a distal apical cell 

 and widens out to form a thalloid plant body with distinctive features 

 as compared with the typical heart-shaped prothallus. The neck of 

 the archegonium, which arises on the lower side, is tilted towards the 

 apical end of the prothallus. The first division of the zygote is by a wall 



