39.2 HOFMEISTER, ON 



the bursting of the macrosporangia. A circular simple 

 cellular layer appears spread over the inner side of the pri- 

 mary spore-membrane, underneath the point in which the 

 four special-mother-cells of the spores touch one another. 

 Those cells are of the greatest height which are situated in 

 the middle of the cellular layer underneath the point of 

 junction of the three projecting ridges of the outer spore- 

 membrane; they divide very soon by transverse septa. 

 Towards the periphery the cells gradually diminish in height ; 

 the outermost have the form of a procumbent wedge (PI. 

 LVII, fig. 16). In many species, especially JS. Jiortensis 

 and helvetica, the rudiment of the prothalhum when seen 

 from above appears to have no distinct boundary, inasmuch 

 as the outermost edge of the marginal cells formed by the 

 convergence, at a very acute angle, of the upper and under 

 wall of the cell, does not refract transmitted light much 

 more powerfully than the primary wall of the spore itself. 

 The marginal cells of the prothallium when seen from above 

 appear open towards the outer side (PI. LVII, fig. 17).* 

 The prothallia of other species, for instance of S. Martensi, 

 exhibit nothing of the sort (PI. LVII, fig. 18). 



I have not yet made out the first stages of development 

 of the rudiment of the prothallium. It is uncertain 

 whether it is formed, like the prothallium of Marsilea, by 

 repeated bipartition of a single cell, or whether, like 



* I believe that the above explanation is sufficient to explain the peculiar 

 phenomenon. Mettenius ('Beitr. zur Botanik,' Part 1, p. 10), deduces from 

 it a mode of development of the cells of the prothallium, which would differ very 

 widely from all other phenomena hitherto observed in the vegetable kingdom. 

 He believes that the prothallium originates between two lamellae of the wall of 

 the spore, which separate from one another; that it increases gradu- 

 ally in circumference whilst those lamella? become further separated from one 

 another, and that new cells are added to the circumference of the prothallium 

 in a manner which — even if it is not yet fully investigated — offers no points of 

 resemblance with the hitherto better known forms of cell-formation. I consider 

 Mettenius' s conclusions to be incorrect, especially because the small-celled por- 

 tion of the prothallium from which the archegonia are produced occupies when 

 fully developed (PI. LVIII, figs. 1, 4), no relatively greater portion of the cir- 

 cumference of the spore than it does when it first becomes visible. I consider 

 it much more probable that the empty cells figured by Mettenius in fig. 10 of 

 the first plate of his work, should be cells, which, by the development of the 

 large-celled inner portion of the prothallium have been pressed against the 

 outer spore-membrane, and squeezed together so as to obliterate the cavity, 

 than that they should be cells in a formative condition. 



