94 EMBRYOGENESIS IN PLANTS 



considerable regularity as the embryo enlarges, Fig. 22b. With these 

 developments the embryo becomes deeply immersed in the tissue of 

 the prothallus, Fig. 22c. On its further enlargement a gradient of cell 

 size from base to apex becomes apparent. The hypobasal tier forms 

 the parenchymatous foot, while the epibasal tier becomes organised as 

 a distal meristem. An older embryo is typically curved and elongated 

 and eventually bursts through the prothallus in an upward direction. 

 Fig. 22d, At its distal end there is an apical meristem where leaf 

 primordia are formed in a spiral sequence. The cells of the suspensor 

 and foot have now become conspicuously enlarged, presumably as a 

 result of the entry of osmotically active substances from the prothallus. 

 The first root becomes differentiated laterally above the foot, Fig. 22d. 

 An incipient vascular strand of elongated cells now traverses the tissue 

 between the shoot and root apices, but no strand is present in the large- 

 celled foot. 



In L. phJegmaria, Fig. 22h, the embryonic cell is divided unequally 

 by a somewhat obliquely transverse wall. The larger distal, or epibasal, 

 cell now divides by a vertical wall, and, a little later, so also does the 

 hypobasal cell. The embryo now consists of four quadrants and an 

 octant stage almost certainly follows. The lower tier gives rise to the 

 foot, the upper to the first leaf and the apical meristem. Rather later, 

 the first root is formed at the base of the leaf quadrant. 



LYCOPODIUM CLAVATUM AND L. ANNOTINUM 



The prothalli are subterranean, saprophytic and massive, and 

 accumulate considerable quantities of storage materials. The initial 

 stages in the embryogeny are as in L. selago. Fig. 22e-g, but soon the 

 foot, which develops in close relation to the sources of nutrition in the 

 prothallus, becomes conspicuously enlarged — a result both of cell 

 division and cell distension. Fig. 27c. This development suggests that 

 the path of nutrients from the prothallus is by way of the suspensor and 

 foot to the apex of the embryo and that considerable quantities of 



Fig. 22. Embryogeny in Lycopodium 



A-D, L. selago. A, Young embryo in l.s. showing the first transverse wall, I-I, 

 which divides the zygote into the shoot segment, s, and the suspensor segment, su, 

 the embryo being endoscopic. B, An older embryo in l.s. showing the orderly 

 segmentation pattern, the position of the first, second and fourth walls, the con- 

 spicuous suspensor, and the adjacent foot region, f. C, An older embryo, with 

 the shoot apex directed inwards, lying within the tissue of the prothallus, p; the 

 embryo has elongated and the foot is now evident. D, Well-developed embryo, 

 with shoot apex, first leaf, incipient vascular strand and large foot. E-G, L. clava- 

 tiim. Stages in the embryonic development as seen in l.s. ; e, the embryonic or distal 

 cell, directed away from the neck of the archegonium (see also Fig. 27C). (A, B, 

 X 240; C, D, x 96; E-G, x 150; after Bruchmann). H, L. phlegmaria. Embryo 

 in l.s., showing segmentation pattern (x 310, after Treub). 



