350 PART IV. CLASSIFICATION. 



structure, and in many cases it does not become free from the 

 spore. It usually lives through but one short period of growth. 



In any one of the homosporous forms, the prothallia developed 

 from the spores are all essentially alike ; generally speaking, any 

 one prothallium bears both male and female reproductive organs. 

 The morphology of the prothallium varies widely in these forms : 

 it may be a branched cellular filament (some Hymenophyllacese), 

 or a flattened expansion (Equisetinse, most Ferns), containing 

 chlorophyll abundantly ; or it is tuberous (Ophioglossacete, Lyco- 

 podiacese), either wholly or in part destitute of chlorophyll. It 

 becomes entirely free from the spore. 



In the heterosporous forms the gametophyte is represented by 

 two individuals a male and a female prothallium ; the former is 

 the product of the germination of a microspore, the latter of the 

 germination of a macrospore. As compared with those of the 

 homosporous forms, the prothallia of the heterosporous forms are 

 relatively small ; moreover they do not become independent of the 

 spores from which they are developed. The male prothallium is 

 reduced to little more than a single male organ (antheridium) ; the 

 female prothallium is a small, usually green, cellular body pro- 

 jecting more (e.g. Salvinia) or less (e.g. Selaginella) through the 

 ruptured outer coat of the macrospore. 



Generally speaking, the symmetry of the prothallium is dorsi- 

 ventral ; in the free-growing forms, the under surface generally 

 bears numerous unicellular root-hairs. The distribution of the 

 sexual organs on the prothallium varies ; they are frequently 

 confined to one surface, but are occasionally scattered over the 

 whole surface. The number of the sexual organs on a pro- 

 thallium is in some cases only one, in others it is consider- 

 able. 



The sexual organs are antheridia (male) and archegoma (female). 

 The structure of the antheridium is simple ; it consists of a wall, 

 a single layer of cells, enclosing the mother-cells of the spermato- 

 zoids. The antheridia are developed from single superficial cells 

 of the prothallium ; when the prothallium is thin, the antheridia 

 project on the surface ; when the prothallium is tuberous, the 

 antheridia become sunk in the tissue. 



The archegonium consists of a venter and a neck. As the 

 venter is, in all cases, sunk in the tissue of the prothallium, it 

 has no proper wall of its own, and is, in fact, simply a cavity 

 in the tissue ; the short neck consists of a single layer of cells 



