CRYPTOGAMS 



357 



similar. It is one of the most beautiful of the free-floating, fresh-water Algae, the 

 hollow cylindrical colonies being formed of elongated cells united together to form 

 a many-meshed net. 



Order 3. Ulotrichales. The Ulotrichales exhibit, as compared with the uni- 

 cellular green Algae, an advance in the external segmentation of the thallus. It 

 is always multicellular, and, in most of the genera, consists of simple or branched 

 filaments. The filaments are either attached by a colourless basal cell to the 

 substratum (Fig. 290 A) or float free. The thallus of the marine genus Ulva 

 (Ulxa lactuca, SEA LETTUCE) has the form of a large, leaf-like cell surface, and is 

 two layers of cells thick (Fig. 5, p. 13). In Entcromorpha (Fig. 289) the thallus 

 is ribbon-shaped, either cylindrical or flattened ; when young it is two-layered, but 

 later it becomes hollow, the wall thus consisting of one layer of cells. Although 

 the majority of the Ulotrichales 

 live in fresh or salt water, a few 

 aerial forms (Chroolepideae) grow on 

 stones, trunks of trees, and, in the 

 tropics, on leaves. To this family 

 belongs Trentepohlia (or Chroolepus) 

 Jolithus, often found growing on 

 stones in mountainous regions. 

 The cell filaments of this species ap- 

 pear red on account of the hsemato- 

 chrome they contain, and possess 

 a violet-like odour. 



The asexual reproduction is 

 accomplished by the formation of 

 ciliated swarm - spores. Sexual re- 

 production is effected either by the 

 fusion of planogametes, or the sexual 

 cells are differentiated as non-motile 

 egg-cells and motile spermatozoids. 



Ulothrix zonata ( x ) (Fig. 290 

 A) is one of the commonest fila- 

 mentous Algae. The filaments of Ulo- 

 thrix exhibit no pronounced apical 



growth; they are unbranched, attached by a rhizoid cell, and consist of single rows 

 of short cells ; each cell contains a band-shaped chloroplast. The asexual reproduction 

 is effected by means of swarm-spores, which have four cilia (C), and are formed by 

 division in any cell of the filament. The swarm-spores escape through a lateral 

 opening (B) formed by absorption of the cell wall, and, after swarming, give rise to 

 new filaments. The sexual swarm-cells, or planogametes, are formed in a similar 

 manner by the division of the cells, but in much greater numbers. They are 

 also smaller, and possess only two cilia (E). In other respects they resemble the 

 swarm- spores, and possess a red eye-spot and one chromatophore. By the con- 

 jugation of the planogametes in pairs, zygotes (F-H) are produced, which, after 

 drawing in their cilia, round themselves off and become invested with a cell wall. 

 After a shorter or longer period of rest the zygotes are converted into unicellular 

 germ plants (/), and give rise to several swarm-spores (K), which in turn grow 

 out into new filaments. Under some conditions the planogametes can give rise 

 to new plants parthenogenetically without conjugating'. Further, the filaments 

 can, in addition to the swarm-spores with four cilia described above, produce 



2 A 2 



FIG. 289. Enteromorpha compressa. (i nat. size.)'. 



