138 Ch lorophyeese 



brought about at any time by employing the requisite combination of the 

 operative factors. This may be partially or wholly true of many other Green 

 Algae ; and since it is well known that the average Green Alga is exceedingly 

 sensitive to external influences, it is not improbable that various combinations 

 of the external factors really determine the successive phases of its life- 

 history. There are, however, among the different groups of Algse, a number 

 of known cases in which certain successive phases are obligatory, and over 

 which the external factors have little control. Among the Chlorophycese 

 these examples are relatively very few. 



Certain post-sexual phenomena of an obligatory character occur in the 

 Ulotrichacese, the Conjugatse, the CEdogoniales, and in some other groups of 

 the Chlorophyceae, notably in the Coleochastacese. 



In Ulothrix zonata the quiescent zoogonidium grows into a typical 

 Ulothria-filament, but the zygospore develops into a dwarf plant quite different 

 in appearance from the normal one. This dwarf individual then gives origin 

 to a brood of zoogonidia, each of which ultimately grows into a typical filament. 

 Thus there is in this Alga what may possibly be a first indication of an 

 alternation of generations, but in the absence of all knowledge of the cytology 

 of the phenomena such an interpretation must be entirely of the nature of a 

 surmise. 



In one or two of the Conjugatse it has been ascertained that a twice- 

 repeated division of the nucleus follows immediately on the sexual fusion in 

 the zygote. Of these four nuclei, two are large and two are small, and one 

 large and one small one go into each of the two cells resulting from the division 

 of the zygote. The smaller nuclei gradually disappear, and although there is 

 no positive evidence of reduction of chromosomes, yet as half the nuclei are 

 discarded it is possible that the phenomenon is a reduction process. 



The zygote of (Edogonium divides into four cells on germination, each of 

 which becomes a ciliated zoospore. In some species it has been demonstrated 

 that these zoospores produce only small asexual plants, and even several 

 generations of asexual plants may be formed before the next sexual plant. 

 This fact has given rise to the suggestion that there is in (Edogonium a 

 rudimentary sporophyte generation, which is comparable to and homologous 

 with that of the Archegoniatse. That this is the case is very doubtful even 

 though the so-called sporophyte occupies the same relative position in the 

 life-cycle. It is quite possible that the division of the zygote into four may 

 represent a chromosome reduction, although as yet there is no positive 

 evidence of this. 



A still more interesting case is that of Coleochtvte. In this Alga a period 

 of rest follows nuclear fusion in the zygote, and the latter on germination 

 divides to form octants, and further divisions result in some cases in as many 

 as 32 cells. Each of these cells becomes (or gives origin to) a zoospore of 



