OF FERNS FROM THEIR SPORES. 119 



elongated in like manner and again divided, and sometimes this goes on until a row of 

 five or. six cells is formed (figs. 3 & 4) : when the first cell grows out into a long filament, 

 fewer cells are formed in the simple row. Chlorophyll granules show themselves, in- 

 creasing in quantity in the newly -formed parts. The first formation of radical fibres often 

 occurs in the earliest stages, consisting in the growth outward of the wall of one or more 

 of the cells of the filamentous prothallium into a slender tube, which attains a great 

 length, remaining narrow and uniform in diameter, and never having its cavity cut up by 

 partitions. All the roots met with on full-grown prothallia exhibit the same characters ; 

 they are tubular prolongations from the inferior walls of cells of the green, vegetating, 

 frondose expansion, and their tubular cavities are freely open into those of the cells from 

 which they arise. 



After a time the youngest cell of the growing prothallium becomes more expanded in 

 the transverse diameter, and after the next transverse subdivision of the cavity we find a 

 new mode of increase, namely, a division of the newest cell in a direction parallel to the 

 original direction of growth ; by the frequent repetition of these two modes of extension 

 the prothallium gradually acquires a somewhat three-sided figure, with the angles rounded 

 off (figs. 5, 6 & 7). When it has attained a certain size a difference begins to present itself 

 in the degree of expansion of the new cells continually formed by subdivision ; those in 

 the middle of the front border (that directly opposite to the original point of growth) 

 remain small and are greatly surpassed in size by those at the sides and at the two ante- 

 rior angles, so that the latter advance forward as rounded lobes, leaving a notch or exca- 

 vation in the middle, giving the entire prothallium the form which in leaves, &c. is termed 

 obcordate (fig. 9). In the meantime great quantities of radical hairs are developed from 

 the lower faces of the cells in the neighbourhood of the obtuse apex of the heart-shaped 

 frond, that is, about the posterior part of the prothallium, which generally exhibits a very 

 ill-defined margin, as the cells formed at first often decay away and disappear, leaving a 

 ragged edge (figs. 9 & 10). 



"While the cordate form, which varies a good deal in different species in the lateral ex- 

 pansion of the two lobes, is becoming perfected, the middle and posterior region of the 

 prothallium, near where the roots arise, begins to display a new mode of growth. Up to 

 this time the entire prothallium consists of a single flat layer of cells, in which state the 

 lateral lobes and the anterior border persist, but in the central and posterior part the 

 cells now become divided by horizontal walls, so as to give the prothallium a thickness 

 of two, three, or more cells in the vertical section (figs. 62 & 67). The thickened portion 

 forms a rounded cushion-like projection from the inferior face of the prothallium (fig. 51), 

 while the upper surface remains flat or is even slightly depressed in the region over the 

 thickening. 



The characters presented by the cells are as follows : — their walls are delicate and closely 

 in apposition, so as to leave no intercellular passages between them ; they are lined by a 

 layer of mucilaginous consistence (the primordial utricle) enclosing a clear fluid filling the 

 cavity of the cell. This substance is coagulated and contracted by acids and iodine, so as 

 to collect all the cell-contents into an isolated mass in the centre. In the cell are also 

 found, more or less abundant, chlorophyll globules, some imbedded in the mucilaginous 



r2 



