REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



244 



of which a structure is formed composed of 

 a series of cylindrical cells placed end to 

 end. The growing extremity of this body, 

 the so-called suspensor, penetrates the single 

 layer of cells which separate it from the in- 

 ferior surface of the prothallium, and buries 

 itself in the cushion^like mass. A new deve- 

 lopment now commences in the terminal 

 cell, which is divided by a succession of septa 

 inclined alternately in opposite directions. 



176, 



Egg-shaped rudiment of embryo of the same attached 

 to the suspensor, from the terminal cell of which 

 it has originated, 300 diam. 



From this results an egg-shaped body, the 

 primary axis of the embryo, which, as it en- 

 larges, 'causes the absorption of the cells by 

 which it is immediately surrounded. Soon 

 after a new (secondary) axis is developed, 



Fig. 177. 



Section of contents of upper part of macrospore of 

 the same, 120 diam. 



a, prothallium, continuous at its periphery with b, 

 the original inner membrane of the spore ; c, 

 cellular body subjacent to prothallium, which 

 projects into the cavity of the spore; a, com- 

 mencement of secondary axis of growth of the 

 embrj'o ; below p are observed the remains of the 

 archegonium within which the embryo origi- 

 nated. Several other unfructified archegonia are 

 seen in section. 



the direction of which (obliquely upward), is 

 nearly opposite to that of its predecessor. 

 It finally makes its escape from the cavity of 

 the spore by penetrating the prothallium near 

 its centre, bearing upon its summit the first 

 pair of leaves of the young plant. 



94-. Sporangia and spores. The sporangia 

 of Selaginella denticulata are formed in the 

 axils of the leaves of the fertile branch, in the 

 following manner. A superficial cell of the 

 stem, the position of which is always imme- 

 diately above the middle of the line of in- 

 sertion of the leaf, is developed to a nipple- 

 shaped projection. The centre of this body 



is occupied by a large cell, which is enclosed 

 by a single layer of others, and supported 



Fig. 178. 



Two germ plants of Selaginella Martensi, which have 

 taken their origin from a single macrospore, 5 diam. 



on a short pedicle. As it advances towards 

 maturity, the spore-case consists of a capsule 

 of three layers. Of these the external or 

 epidermic, is composed of narrow prismatic 

 cellules containing only a transparent fluid. 

 The cells of the middle layer are tabular, 

 and contain starch granules, while those most 

 internal are narrow and somewhat columnar, 

 with very delicate walls. Within this capsule 

 is enclosed a central mass of larger cells, 

 which exhibit central nuclei and granular 

 contents. These, which are the parent cells 

 of the spores, are at first intimately united, 

 but afterwards lie loose in the cavity of the 

 spore-case. Up to this point the development 

 of all the sporangia is uniform. In those 

 in which macrospores are to be produced 

 (oophoridia), one of the parent cells, in no 

 respect different from its fellows in struc- 

 ture, continues to increase in size while they 

 disappear. Its nucleus is soon replaced 

 by four others, which arrange themselves, as 

 in Equisetum, towards the four angles of a 

 regular tetrahedron. Septa are afterwards 

 formed, which divide the cell into four com- 

 partments, in each of which a spore is de- 

 veloped. The spore at first exhibits only a 

 delicate membrane, but as it approaches 

 maturity the three converging ridges, and, 

 finally, the external tegument, the structure 

 of which has been already described, are 

 formed upon its surface. No sooner is this 

 process completed than the membrane of 

 the parent cell disappears, the four spores 

 retaining their relative position, however, to 

 each other, apparently attached by the 

 remainder of the septa. It is at the point 

 at which the spores are in relation with 

 the centre of the mother cells, that the three 

 ridges converge, as well as the three lines by 

 which the valves of the external tegument 

 give way to allow of the growth of the pro- 

 thallium. 



95. In those sporangia in which microspores 

 are to be formed, all of the original parent 

 cells exhibit a development which corresponds 

 with that which is above described as occur- 

 ring in one only in the oophoridium, with this 

 exception, that they do not attain the same 

 dimensions. Hence results a large number of 

 microspores which resemble the macrospores 

 in the structure of their internal membrane, 

 and external three-valved tegument, but differ 



