CH. XLix] ALTERNATION 265 



new gametophyte. If these functions be carried out in the uniform medium 

 of water, as in the Algae, the two somata may be and often are alike: in 

 them syngamy is commonly effected 'through the medium of water, and it 

 can happen at any time, while the carpospores that are its ultimate result 

 float naturally away to new stations under conditions that are uniform, 

 discharged by generations that may be uniform also. Thus in the Algae 

 the problems of propagation and dissemination are easily solved. In all 

 primitive land-plants we iind s}-ngamy through the medium of water rigidly 

 maintained, and no normal cycle is complete without it : but for them the 

 time and place of this regularly recurrent syngamy are restricted, since 

 external liquid water is not constantly available. On the other hand, the 

 carpospores, when detached, are spread through the medium of air: they 

 must as a rule be scattered as dry dust if new stations are to be taken up. 

 Thus in the Archegoniatae, which may be described as the amphibians of 

 the Vegetable Kingdom, the one critical event of the life-cycle depends on 

 water, the other on air, for its effective discharge. 



The regularly stabilized alternation, and the marked difference in size, 

 form, and structure of gametophyte and sporophyte are best interpreted in 

 relation to these conflicting requirements. In the Filicales the gametophyte 

 is relatively small, and usually evanescent: as a rule it bears gametangia 

 early, and after producing one embryo it dies: but that embryo is stationary 

 at the spot where it was produced. The sporophyte develops as a perennial 

 plant, with a much more definite hold upon the soil, and an elaborate aerial 

 shoot : this is able to produce successive crops of spores : each spore may 

 be air-borne to a distance, and in favourable circumstances will germinate 

 in its new station. The divergent features of the two somata and their con- 

 stant alternation are clearly adapted to this separation of the functions of 

 syngamy and dissemination, conducted as they are in different media. Their 

 diff"erence may be held as a natural consequence of the amphibial life, which 

 has guided a less standardised alternation into a special and constant channel, 

 as now seen in the Archegoniatae generally and in the Ferns in particular. 



The interest in the two alternating generations or somata centres in the 

 propagative organs which they bear. Von Goebel has suggested the propriety 

 of adopting a uniform terminology for them : on the one hand gametangia 

 are borne upon the gametophyte, and are liable on sexual differentiation to 

 appear respectively as viega-ganietangia (archegonia), and jnicro-gainetangia 

 (antheridia) : on the other hand sporangia are borne upon the sporophyte, 

 and though all are primitively alike, they are liable on sexual differentiation 

 to appear as mega-sporangia and micro-sporangia (Vol. I, p. 290). The origin 

 of the two types of sporangia from a common source is a familiar topic 

 of the textbooks: the common origin of the gametangia is less obvious, 

 being suggested by detailed comparison rather than by demonstration 



