LESS, xxix THE PROTHALLUS 357 



rhizoid (rJi) springing from the proximal cell of the rudimentary (two- 

 celled) prothallus. 



D, a young prothallus, showing spore, rhizoid (rJi), apical cell 

 (ap. c), and segmental cells (seg. c). 



E, an advanced prothallus, from beneath, showing rhizoids (rh}, 

 ovaries (ovy), and spermaries (spy}. 



F, a mature spermary of Pteris, inverted (i.e. with its distal end 

 directed upwards) so as to compare with Fig. 82, A. 



G, a single sperm, showing coiled body and numerous cilia. 



H, a mature ovary of Aspidium, inverted so as to compare with Fig. 

 82, B 2 , showing venter (v), neck (#}, ovum (ov), and canal cells (en. c). 



I, small portion of a prothallus of Asplenium in vertical section, 

 showing the venter {v) and part of the neck (n) of a single ovary after 

 fertilization. The venter contains an embryo just passing from the 

 polyplast into the phyllula stage, and divided into four groups of cells, 

 the rudiments respectively of the foot (ft], stem (st), root (rt), and 

 cotyledon (ct}. 



K, vertical section of a prothallus (prtJi) of Nephrolepis, bearing 

 rhizoids (r/i), and a single ovary with greatly dilated venter (v) and 

 withered neck (). The venter contains an embryo in the phyllula 

 stage, consisting of foot (ft}, rudiments of stem (.$/), and root (rt}, and 

 cotyledon (ct} beginning to grow upwards. 



L, prothallus (prth} with rhizoids (r/i), bearing a young fern plant, 

 consisting of foot (//), rudiment of stem (st), first root (rt), cotyledon 

 (ct), and first ordinary leaf (/). (After Howes.) 



only one layer of cells thick, but it gradually increases in 

 size, becoming more or less kidney-shaped (E), and as it does 

 so its cells divide parallel to the surface, making it two and 

 finally several cells in thickness. Thus the prothallus is 

 at first a linear, then a superficial, and ultimately a solid 

 aggregate. Root-hairs (rh) are produced in great number 

 from its lower surface, and penetrating into the soil serve 

 for the absorption of nutriment. At an early period a two- 

 sided apical cell (D, ap. c) is differentiated, and gives off 

 segmental cells (seg. c) in the usual way : an abundant forma- 

 tion of chromatophores also takes place at a very early period 

 in the cells of the prothallus, which therefore resembles both 

 in structure and in habit some very simple form of moss. 



On the lower surface of the prothallus gonads (E, spy, ovy) 

 are developed, resembling in their essential features those of 



