REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



spore membrane, and consequently formed 

 before it, line the inner surface of the parent 



Fig. 



173. 



Development of parent cells of the spores of the same- 



a, one of the nucleated cells which constitute the 

 central mass of the young sporangium ; b, the 

 central nucleus has disappeared, and is replaced 

 by four others, one of which is out of focus; 

 c, the cell is divided by six septa into four 

 somewhat tetrahedral compartments. This ob- 

 ject has by mistake been represented relatively 

 smaller than the rest; d, the four compartments 

 (special parent cells of the spores), are about to 

 separate from each other : e, mature special 

 parent cell. In its interior we observe the nu- 

 cleated spore, and between it and the membrane 

 of the parent -cell, the coils of the two elastic 

 fibres ; f, the free spore ; the spiral fibres, which 

 remain for a short time after its escape from the 

 parent-cell, attached by their middle points to its 

 membrane, have disappeared. 



cell, from which the gelatinous thickening 

 has now disappeared. Soon after, springing 

 asunder from each other, they tear the 

 membrane of the parent cell, retaining, how- 

 ever, their central attachment to the surface 

 of the spore. 



90. LycopocKacees.* The large spore (ma- 

 crospore) of Selaginella, consists, when ripe, 

 of an internal spherical vesicle of delicate 

 structure (endospore), which is enclosed in 

 a resistant epispore. The endospore contains 

 a fluid, in which float mucous and oleaginous 

 granules only, its nucleus having disappeared. 

 On its surface are observed three linear 

 projections, all of which converge towards 

 one point, the summit of the spore. The 

 epispore, a structure of later formation, is 

 composed of two layers, the internal of 

 which is distinguished from the other by its 

 remarkable transparency. The external sur- 

 face is scattered over with acuminated projec- 

 tions, which are connected with each other by 

 a network of minute ridges. 



91. The development of the prothallium 

 commences (usually several months after the 

 macrospore has been sown) by the deposi- 

 tion of several cells on the internal surface 

 of the proper spore membrane, at a point 

 subjacent to that towards which the three ex- 

 ternal ridges converge. Whether these cells 



* Hofmeister, /. c. pp. 118124. 



243 



are originally developed in silu, from a single 

 parent, or existed before, lying free in the 

 cavity of the spore, is uncertain. Hofmeister 

 inclines to the latter opinion. At first the 

 prothallium is a cellular expansion of circular 

 form, which enlarges by growth at its periphery, 

 and lines the upper part of the proper mem- 

 brane of the spore. At its centre it is of 

 considerable thickness, and is composed of 

 several layers of cells. Towards its margin 

 it becomes gradually thinner and thinner, its 

 two surfaces at last converging at a very acute 

 angle, so as to become continuous with those 

 of the spore membrane. 



92. Arckegonia. The first-formed arche- 

 gonium is always found to occupy the centre 

 of the upper surface of the prothallinm ; its 

 successors surrounding it at various distances. 

 A superficial cell, distinguished from its 

 neighbours by the quantity of granules which 

 it contains, is divided into two by a trans- 

 verse septum. From the upper of the resulting 

 compartments is developed a papilliform 

 projection, which is composed, as in the Equi- 

 setaceae,of two double pairs of cylindrical cells, 

 an axial intercellular canal. In 



surrounding 



Fig. 174.. 



Section of unimpregnated archegonium of Selaginella 

 dentieulata, containing the granular germ-cell, 400 

 diain. 



the lower is contained a vesicular nucleus, 

 the germ-cell of the fully formed archegonium. 

 Its cavity becomes continuous with the 

 axial canal by the solution of its membrane." 



Fig. 175. 



Archegonium of the same immediately after impregna- 

 tion, GUO diam. 



The germ-cell has enlarged considerably, and di- 

 vided by a transverse septum. 



93. The embri/o. About the time that the 

 formation of archegonia is completed on the 

 upper surface of the prothallium, there is 

 developed, on its inferior aspect, a tissue 

 composed of cells much larger than any of 

 those previously existing. This tissue pro- 

 jects, in the form of a cushion, into the cavity 

 of the spore. In general one only of the many 

 archegonia receives the necessary fructifying 

 influence. In this the germ-cell divides re- 

 peatedly by transverse septa, as the result 



R 2 



