68 EIGHTEENTH REPORT. 



the point in the cycle indicated by G) produce a diploid zygote 

 which develops into the plant body without any meiotic division 

 until the eggs and sperms are produced. Thus the three events 

 of the cycle occur in immediate proximity in the order ED, CU, 

 NU. 



In Ulothrix and Oedogonium and a number of other Chlorophy- 

 ceae the life history includes the production of gametes, their 

 union to zygospores or oospores, and the germination of these, after 

 a longer or shorter period, by the division of the nucleus into four 

 nuclei and the production of four zoospores, each of which pro- 

 duces a new plant. (In Ulothrix according to Klebs, these four 

 cells may possibly lack motility). If conjugation is prevented the 

 gametes of Ulothrix are capable of developing parthenogenetically. 

 These facts lead to the assumption (which ought to be tested by 

 cytological investigations) that the vegetative cells and the gametes 

 of these plants are haploid. That being the case the ability of the 

 gametes to grow without conjugation would not seem strange. The 

 division of the zygote into four cells is probably accompanied by 

 the meiotic divisions of the nucleus. The sexual cycle would then 

 be illustrated by a diagram (Fig. 4) in which the events are in 

 the order, CU, NU, IJD with the main part of the cycle in the hap- 

 loid condition. 



It is probable that the Desmids and Pond Scums (Zyguema- 

 tales) are of this same type, for we find the zygote dividing into 

 four cells in Mesotaenia or into two cells with two nuclei each 

 (and one of these two disintegrating) in most Desmids, or, as in 

 Spirogja'a, with the zygote nucleus dividing into four nuclei, with 

 only one finally functioning further. It seems almost certain that 

 the reduction divisions must occur at this point, but here, too, the 

 matter needs further investigation. 



In the higher algae we find that in the Florideae the sexual cycle 

 shows another modification. (Fig. 5). In the majority of this class 

 two generations are distinguishable, the sexual and the tetrasporic. 

 The zygote nucleus divides by ordinary mitotic divisions and is 

 seen to be diploid. The resulting nuclei may invade other cells or 

 not, but eventually enter the threads that give rise terminally to 

 the carpospores. These are also diploid as are the cells of the 

 tetrasporic ])lant arising from them. It is worthy of note that this 

 generation with the diploid nuclei consists of plants as a general 

 rule much larger than the haploid sexual plants of the same age.* 



*A similar difference in size exists V)etween Oenothera lamarckiana and one of of its mutants, 

 Oe. gigas and it is worthy of note that the latter has double the number of chromosomes pos- 

 sessed l).v the former. 



