;6 ALTERNATION IN THE THALLOPHYTES 



The criterion of chromosome-number is new : the method of physiological 

 experiment is also new. Still, the conclusions are in the main unaltered. 

 What was then styled " homologous alternation " now stands on a basis 

 of cytological unity as regards the somatic divisions, and denotes such 

 recurrent phases in plants as appear to be dependent on external condition, 

 not obligatory in their succession, and involve no cytological change : 

 this includes the cases grouped under (i) and (2) above. There is hardly 

 any need to designate such life-cycles as showing alternation at all, were 

 it not that this is the type of life-history for which the term was first 

 introduced by the zoologist Steenstrup. The types grouped above under 

 the heading (3) were distinguished by Celakovsky as showing "antithetic 

 alternation," and it is now found to have its basis in a cytological difference 

 of the successive phases, which also show an obligatory succession, not 

 determined directly by external conditions. 



The conception of normal antithetic alternation now turns upon the two 

 critical points of sexual fusion and reduction : it is necessary to enquire how 

 far these events are historically the same in organisms at large. It would 

 seem probable that sexual differentiation, and perhaps even sex itself 

 originated along several distinct phyletic lines : on this point there is no 

 definite information, though the differences of character of the organisms 

 which show the simplest types of sexuality distinctly suggest that it had 

 not one common source only. In the present state of uncertainty it seems 

 .undesirable to depart from the usual convention by which the zygote is 

 held to be "homologous"; and, accordingly, it serves as a point for general 

 comparison between representatives of distinct phyla. But it must be 

 distinctly understood that this is in itself a conventional understanding, and 

 that its adoption for convenience of description does not necessarily imply a 

 strict "homogeny, " in the sense that sexuality was established once for 

 all. Similarly with reduction, which is theoretically a necessary consequence 

 of sexual fusion, it is only by a similar conventional understanding that 

 in divers organisms the cell where this is initiated is held to be "homologous": 

 it is not to be assumed that it is truly "homogenetic" in distinct phyla, 

 as though reduction had been initiated once for all in sexual organisms. 

 But, on the other hand, in organisms that are akin, such as the members of 

 the phyla of the Ferns or the Mosses, it may reasonably be held as probable 

 that the zygote and the spore-mother-cell are actually identical things, in fact 

 homogenetic for the whole phylum, in the sense that each probably sprang 

 from a phyletic source common for the whole phylum. 



A comparison of plants at large as regards the position of the reducing 

 process in the life-cycle relative to sexual fusion shows great differences, 

 as we have seen. It is not improbable that these may have been due 

 in part to initial differences : we have no right to. assume that there was 

 uniformity at the outset. Some ground for the view that initial differences 

 existed is to be found in such cases as the Desmids and Diatoms ; for in 

 Closterium the rejection of the superfluous nuclei, and probably also 



