198 THIRD GROUP. VASCULAR CRYPTOGAMS. 



even to that of the protonema in some Mosses. The prothallium produces simple, 

 tubular, unsegmented rhizoids, and ultimately antheridia and archegonia. Its de- 

 velopment and duration may occupy a considerable space of time, especially if the 

 oospheres are not fertilised. 



If the conditions, especially of moisture and warmth, are favourable, the spores 

 begin to germinate in a few days after they are sown. Most fern-spores retain their 

 vitality a long time ; the spores only of the Osmundaceae and Hymenophylleae, which 

 contain chlorophyll as soon as they are fully formed, soon lose the power of germ- 

 ination. Other fern-spores require a longer or shorter period of rest before they 

 germinate. The admission of water causes the contents of the spore to swell and 

 burst the cuticularised exosporium, which is usually provided with ridges, tubercles, 

 spikes or granulations along its edges, if there are any; in bilateral spores the 

 exosporium opens by a longitudinal fissure. In suitable objects, such as the germin- 

 ating spores of the Gleicheniaceae, it may be seen that when preparing for germ- 

 ination the contents of the spore become invested with a new cellulose-membrane J , 

 and this proceeding appears to be quite general. Then the newly formed membrane 

 protrudes in the form of a papilla from out of the opening in the wall of the spore, 

 chlorophyll and other substances make their appearance in the protoplasm, and very 

 soon a second small protuberance forms the rudiment of the first rhizoicl, which like 

 the first cell of the prothallium is cut off from the contents of the spore by a partition- 

 wall. The Polypodiaceae may be chosen to supply examples of the further de- 

 velopment of the prothallium. The papilla which has been described as projecting 

 beyond the wall of the spore first of all developes into a row of cells, the terminal cell 

 in which divides by transverse septa, a proceeding less frequently observed in the 

 other cells of the row. Then the cells towards the end of the row grow broader and 

 the terminal cells also divide longitudinally, and thus the cell-filament becomes a cell- 

 surface of a spathulate form. At its extremity is either a two-sided apical cell, as in 

 Metzgeria, or a group of small marginal cells. But the apical cell, when present at 

 first, is not long maintained ; a periclinal transverse wall is soon formed in it, and this 

 is followed by a number of longitudinal walls in the cells nearest the apex, so that in 

 this case also the apex comes to be occupied by a number of marginal cells. Soon 

 the prothallium changes its shape and becomes reniform or heart-shaped. The 

 growing point lies in the sinus and is composed of a number of meristematic cells ; 

 these cells are plainly distinguished from the rest by their smaller size and the large 

 amount of their protoplasm. 



Behind the growing point the tissue of the prothallium is now composed of 

 more than one layer of cells, and a cushion-like mass of tissue forms behind the sinus, 

 on which the archegonia arise, usually in acropetal succession. From this cushion 

 also, but on another part of the under surface, grow numerous rhizoids (unicellular 

 root-hairs), which secure the prothallium to the substratum. The antheridia are 

 not, like the archegonia, confined to the cushion, but may appear on any marginal 

 or other cells of the prothallium. It is not uncommon, especially when the spores 

 have been sown in great profusion, to find prothallia thickly covered with antheridia, 

 but without an archegonium. These prothallia, moreover, have not the meristem 



1 Rauwenhoff, Ueber d. ersten Keimungserscheinungen der Kryptogamensporen (Bot. Ztg. 1879, 

 p. 441). 



