THE ORIGIN OF SEX IN PLANTS. 



71 



4 or 8 in a cell. They are relatively large structures with 4 cilia and 

 have the appearance shown in Figure 2, c. These 4-ciliate zoospores 

 are never sexual and they develop new Ulothrix filaments like their 

 parent. This simple method of reproduction may be continued for 



Fig. 2. Ulothrix. a, Vegetative Filament. 6, Development of Asexual Zoospores. 

 c, Zoospore, d, Sporelings. e, Cell containing Gametes. /, Gametes, g, Conjugation of 

 Gametes, h. Sexually formed Spore. 



many months, but at times the conditions are such that another form 

 of swarm-spore appears. These elements are much smaller than the 

 usual zoospores, are developed more numerously in the mother cell and 

 have 2 cilia as is shown in Figure 2, e, f. They are gametes and as a 

 rule fuse readily with one another in pairs. The free-swimming 

 gametes are sho'^vn in Figure 2, f, and two stages in the conjugation 

 appear in g and h. If conjugation does not take place, the gametes 

 settle down and in certain instances have been observed slowly ger- 

 minating ; but they develop feeble plants. 



Now what are the causes that make the plant produce asexual zoo- 

 spores on the one hand and gametes on the other? Are they deeply 

 seated in the protoplasm of Ulothrix? In the first place there is no 

 rule or rhythm in the appearance of zoospores or gametes, no time when 

 conditions within the plant demand their development. And again, 

 structurally, there is no hard and fast line between the zoospore and 

 gamete; on the contrary, there are gradual transitions between these 

 two forms of swarm spores. The problem thus resolves itself into an 

 inquiry as to the precise environmental influences, the chemical and 

 physical factors affecting the Ulothrix filament, whether they are ac- 

 tually able to make the plant form zoospores or not according to certain 

 conditions. The habits of Ulothrix show us clearly that there are such 

 factors, but the adjustments are so delicate that, apart from a very clear 

 relation to temperature and the character of the salts in solution, it has 

 not been possible to formulate them with exactness. 



But other studies of Klebs, on forms that lend themselves more 

 readily to cultivation than Ulothrix, have given some very definite re- 

 sults. Hydrodictyon, the water-net, is an alga^ that may be cultivated 



