376 PART III. THE CLASSIFICATION OF PLANTS. 



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 (Equisetinaa, most Ferns), containing 

 chjorophyll abundantly ; or it is tuberous (Ophioglossacese, Lyco- 

 podiaceae), 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 ; the female pro- 

 thallium is a small, usually green, cellular body projecting 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. In some cases the pro- 

 thallium shows more or less well-marked differentiation into a 

 vegetative portion and a gametophore which may bear either both 

 kinds of sexual organs, or, more commonly, one kind only ; when 

 the gametophore bears only male organs it is distinguished as an 

 antheridiophore ; when only female organs, as an archegoniopliore. 

 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 prothallium is in some cases only one, in others it is 

 considerable. 



The sexual organs are antheridia (male) and archegonia (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 



