It will be noted that the two generations are more nearly equal than in the fern.
In the liverworts, the life history is essen tially the same, but the protonema is incon spicuous or entirely lacking. The sporophyte is not so highly developed as in the moss, and, in the lower liverworts, the sporophyte is en tirely included within the gametophyte. But here again the sporophyte begins with the fer tilized egg and the gametophyte begins with the spore.
In plants below the liverworts, i. e., in the alga:and fungi, alternation of generations is easily recognized in many cases. In forms which have not reached the level of sexuality there is no alternation, but with the origin of sex, i. e., with the fusion of gametes, the sporo nhyte had its beginning. In many of these simple plants reduction of chromosomes occurs immediately after the fusion of gametes with out the formation of any spores, no recog nizable body being built up which could be called a sporophyte. As the interval between the fusion of gametes and the reduction of chromosomes became more prolonged, a recog nizable body was formed, which in some cases looked like the gametophyte and in some cases looked different. When this body produces spore mother cells, each of which gives rise to four spores, practically all botanists admit that there is an alternation of generations. It must be admitted that the term sporophyte is a misnomer in those• simple forms which do not produce spores, but the fundamental fea ture is this, that the fusion of gametes initiates a generation — long or short — which comes to a close with the reduction of chromosomes.
Consequently, we should insist that the two generations are marked by the number of chro mosomes, the sporophyte having twice as many as the gametophyte.
The sporophyte, at its beginning, is small and, in many cases, dependent upon the gameto phyte; but higher in the scale it becomes in dependent and is the generation which we recog nize as the plant. On the other hand the gametophyte in the early forms is the domi nant generation which, higher in the scale, be comes dependent, and then more and more reduced until it can be seen only with the aid of the miscroscor. Here it is the sporophyte which we recognize as the plant. This evolu tion of the sporophyte and reduction of the gametophyte are among the most interesting phenomena of plants.
We believe that in animals, as well as in plants, there are two alternating generations, characterized by the number of chromosomes, but this belief is shared by only a few botanists and, as far as we know, practically all zoologists oppose it.
Bower, F. 0., 'The Origin of a Land Flora' (Botanical Gazette, Vol. XXXIX, 1905, p. 137) ; Chamberlain, Charles 'Alternation of Generations in Animals from a Botanical Standpoint' ; Lang, W. H., 'On Apogamy and the Development of Sporangia upon Fern Prothallia) (Philosophical Transac tions, Royal Society of London, Vol. CXC, 1898, p. 187).