DIV. I 



PTERIDOPHYTA 



497 



r 



r 



W 



B, Prothallium with young fern attached to it by its foot ; 

 b, the first leaf; w, the primary root, (x circa 8.) 



sexual generation bears the antheridia and archegonia ; the asexual 



generation develops from the fertilised egg and produces asexual, 



unicellular spores. On 



germination the spores in 



turn give rise to a sexual 



generation. Since the 



reduction division takes 



place on the formation of 



the spores, the sexual 



generation is haploid and 



the asexual generation 



diploid. 



The SEXUAL 'GENERA- 

 TION is termed the PRO- 

 THALLIUM or GAMETO- 

 PHYTE. It never reaches 

 any great size, being at 

 most a few centimetres in 



diameter; in some forms FIG. 46l.~Dryopteris(Aspidium)JUix mas. ^1, Prothallium seen 



it resembles in appearance from ** low '' ar ' arche s nia ; , antheridia ; rh, 

 a simple, thalloid Liver- 

 wort ; it then consists of 

 a small green thallus, attached to the soil by rhizoids springing 

 from the under side (Fig. 461 A). In other cases the prothallium 



is branched and fila- 

 mentous ; sometimes it 

 is a. tuberous, colour- 

 less mass of tissue, 

 partially or wholly 

 buried in the ground, 

 and leading a sapro- 

 phytic existence, in 

 symbiosis with an endo- 

 phytic fungus forming 

 a mycorrhiza, while in 

 certain other divisions 

 of the Pteridophyta it 



Fir,. 46-2. A, Pteris serrulata, embryo freed from the archegonium, Undergoes reduction 



in longitudinal section (after KIENITZ-GERLOFF): 7, basal wall; a nr ] rpmainq mnrp nr 



II, transverse wall dividing the egg-cell into quadrants ; rudi- f 



ment of the foot/, of the stem s, of the first leaf b, of the root w. l6SS com pJetely enclosed 



B, Section of a further-developed embryootPteridiumaquUinum within the Spore. 



(after HOFMEISTER); /, foot still embedded in the enlarged Q n 4.1^ nro thallia ar^P 



venter of the archegonium aw ; pr, prothallium. (Magnified.) ^ 



the sexual organs, 



antheridia (Figs. 468, 475), producing numerous ciliate, usually spiral 

 spermatozoids, and archegonia (Figs. 469, 476), in each of which is 

 a single egg-cell. As in the Mosses the presence of water is necessary 



2K 



