THE DEVELOPMENT OF ORGANISMS 775 



cells is the ovum; the others, often eight in number, are auxiliary 

 "canal-cells" which break down and leave a path by which a 

 spermatozoon may enter. In Marchantia these female organs or arche- 

 gonia occur in the notches of a star-shaped stalked receptacle, whose 

 shape suggests a fairy parasol. By inequality in the growth of the disc 

 the archegonia are bent round to the surface next the thallus, and 

 there they hang with the mouth of the flask downwards. 



When the egg-cell is fertilised, it develops into a small sporophyte, 

 which is anchored in the gametophyte and bears a rounded 

 spore-capsule on the tip of a delicate stalk or seta. The tissue 

 inside the capsule makes spores, but it also forms elongated cells 

 with a spiral thickening (elaters), which are very hygroscopic 

 and by their twisting about when moistened help to scatter the 

 spores. 



As might be expected, there are several grades among the Liver- 

 worts and numerous differences in detail; but the general sequence 

 of the life-history is the same in all, and may be thus represented in 

 schema : 



Spores >■ Sexual > Egg-cells and > Sporcphyies > Spores 



Gametophytes sperm-cells (Diploid) 



(Haploid) 



MOSSES. — ^The large group of Musci is allied to the Liverworts 

 and probably evolved from some of their progressive types among the 

 Anthocerotales or the Jungermannias. They are represented all over 

 the world except in the sea, and they are very successful in all zones. 

 Low-growing, unless epiphytic, very easily multiplied asexually, able 

 to resist both frost and drought, mosses have taken a strong grip 

 of the earth, and are rich in species. They afford shelter to a multitude 

 of minute animals, such as Rotifers and Mites. But we have here to 

 do with their life-history. 



Most characteristic perhaps is the sporophyte which grows up 

 from the fertilised egg-cell in the female organ of the familiar moss- 

 plant. On the tip of a slender stalk there is a very complex little 

 capsule, containing spore-forming tissue; and outside the capsule 

 for a time there is a hood or cap, called the caljrptra. The top of the 

 capsule bears a lid or "operculum" which eventually slips off, and 

 just below the lid there is a beautiful ring of little tooth-like projec- 

 tions, forming the "peristome". In the true mosses no small part 

 of the capsule consists of green photosjmthetic cells; but the upper 

 part is given over to spore formation. Perhaps the green tissue is a 

 hint of a possible independence of the Sporophyte, but this is never 

 achieved in Bryophytes. 



A spore falling on the moist soil develops into a branching fila- 

 ment (the protonema), which is the equivalent of the thallus of a 

 liverwort. From the filament there grow slender branches, some- 



