LESS, xxxi THE PROTHALLUS 



425 



rhizoid (rh} springing from the proximal cell of the rudimentary (two- 

 celled) prothallus. 



D, a young prothallus, showing spore, rhizoid, 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. 109, A. 



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



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

 109, B 3 , showing venter (v), neck (n}, 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 ovai-y after 

 fertilisation. The venter contains an embryo just passing from the 

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

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

 cotyledon (ct}. 



K, vertical section of a prothallus (prth} of Nephrohpis, bearing 

 rhizoids (rh), and a single ovary with greatly dilated venter (v) and 

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

 stage, consisting of foot (ft}, rudiments of stem (st}, and root (rt}, and 

 cotyledon (ct} beginning to grow upwards. 



L, prothallus (prth} with rhizoids (rh), bearing a young fern-plant, 

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

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



distal cells divide longitudinally, a leaf-like body being 

 produced, which is called \hsprothallus (D). This is at first 

 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 

 formation of chromatophores also takes place at a very early 

 period in the cells of the prothallus, which therefore re- 

 sembles both in structure and in habit some very simple 

 form of moss. 



