464 



BOTANY 



then takes place, is followed directly by the segmentation of the fertilised egg-cells 

 and the formation of the embryos (Fig. 427). 



The embryogeny of Selayinella recalls that of Lycopodium. The egg-cell is 

 divided by the formation of a transverse wall into two cells ; the upper and 

 larger cell increases considerably in size, and gives rise, by the division of its 

 lower portion, to a suspensor (Fig. 428 ct), while the lower cell, by repeated 

 division, develops into an embryo, provided with two primary leaves and further 

 .segmented into stem, root, and foot (bl, st, w, f). The foot, in this instance, has 

 a different position and origin than in Lycopodium. Each primary leaf has, even 

 at this stage, a ligule (lig) formed as an outgrowth of the leaf-base. 



The suspensor is perpendicular to the axis of the embryo ; its function is to 

 push the embryo into the tissue of the prothallium, with which the foot, the organ 

 of absorption, is thus kept in close contact: The stem apex, with the first pair of 

 leaves, eventually grows upwards, and the root also extends beyond the macro- 

 spore. As the foot still remains in the prothallium the young plant continues 

 united to the spore, and presents the appearance of a phanerogamic seedling with 

 the seed still attached (Fig. 424 B). 



Order 3. Isoetaeeae ( 1 --) 



The isolated genus Isoetes must be regarded as a persistent branch of an ancient 

 group of plants. It exhibits some affinities with the eusporangiate Ferns. The 



species of Jsoctcs are perennial plants, growing 

 either on damp soil or submerged in water. The 

 stem is short and tuberous, terminating below 

 in a tuft of dichotomously branching roots, 



. 4-_".. Isoetes lacustris. (i nat. size.) 



;. 430. A-F, Isoetes setacea (x 640). A, microspore 

 seen from the side. B-D, segmentation of the spoiv : 

 p, prothallial cell; w, the- fonr cells of the wall ; 

 s. .sjii'Miiato.uenous cells. 7-,', the four spermatozoid 

 mother-cells ure .surrounded by the disorganised oeilfl 

 of the wall ;lsurface view. F, the same in side view. 

 0, Isoetes Malinverniana, spermatozoid. (x 780. 

 After BELAJEFF.) 



and above in a thick rosette of long, stiff, awl-shaped leaves (Fig. 429). The 

 leaves are traversed longitudinally by four air-passages, and expand at the base 

 into a broad sheath. On the inner side of the leaves, above their point of 

 insertion, is an elongated pit, the fovea, containing a large sessile sporangium. 



