118 EMBRYOGENESIS IN PLANTS 



We may now consider how the facts of the embryogeny bear on these 

 very different views. 



Bower (1935) has pointed out that, at first sight, the embryogeny 

 of Isoetes appears to be very different from that of Selaginella, although 

 the parental prothalli are much alike. But (he argues) if it be assumed 

 that Isoetes had once possessed a suspensor like Selaginella, and that 

 this suspensor has been lost or eliminated in the course of descent, 

 then the inversion of the embryo and a consequential exoscopic 

 embryology might in due course follow. The variable obliquely 

 transverse position of the first wall is regarded as being of some 

 theoretical importance in this assumed inversion, i.e. it may be a 

 residual condition associated with the inversion. Otherwise he con- 

 siders that the two tiered octant stage in Isoetes may be compared with 

 the two tiered embryonic region in Selaginella. The loss of the suspensor 

 and the inversion of the embryo in Isoetes, if true, pose genetical and 

 physiological questions of great interest and difficulty. Bower refers to 

 the work of La Motte (1933) who showed that the embryo in Isoetes 

 shifts its position as it grows, the axis constituted by the first leaf and 

 first root lying approximately parallel to the surface of the gametophyte. 

 It should be noted, however, that it is only the extensive growth in the 

 transverse plane that brings about this disposition of the embryo, the 

 direction of the very abbreviated shoot or axis being still that determined 

 at the first division of the zygote. 



As supporting his views on the alfinity of Isoetes, Campbell sees in 

 the organogenic development of the embryo (with its conspicuous, 

 early leaf and root formation, delayed shoot formation, and the 

 relationships of the vascular strands of leaf and root) arrangements like 

 those found in the embryogeny of certain eusporangiate ferns, e.g. 

 Marattiaceae and Ophioglossaceae. 'The embryo of Botrychium 

 virginianum perhaps most nearly resembles that of Isoetes, but there 

 are resemblances to the embryo of the Marattiaceae also.' 



Fig. 29. Embryogeny in Isoetes echinospora and /. lithophila 

 A, The ovum at fertilisation; a, archegonium. B, First division of ovum by an 

 approximately transverse wall. C, Older embryo, with fairly regular divisions; the 

 first wall has been somewhat oblique to the archegonial axis. D, Median l.s. of an 

 embryo, just before the cotyledon (c) breaks out of the prothallus; the foot, /, 

 consists of enlarged cells; %, ligule; r, root. The shoot apex originates in the 

 depression at the base of the ligule. (A, B, x 330; C, x 300; D, x 200; after 

 Campbell.) E, F, G, Diagrammatic representation of the embryonic development 

 in variously orientated gametophytes of /joe/t-i; L, first leaf; /?i, first root; F, foot. 

 H, J, K, L, M, Early stages in the embryonic development in Isoetes lithophila; 1-1, 

 the first partition wall; 2-2, the second wall; I-IV, the quadrants; the position of 

 the archegonial neck is indicated by the two short converging lines and the zenith by 

 Z. N, O, P, Isoetes lithophila, showing the relation of the initial segmentation to the 

 subsequent organogenic development; the finer shading represents the incipient 

 vascular tissue; s, sheath of primary leaf; L^, L^, leaves; st, site of shoot apex; 



Ig, ligule (all after La Motte). 



