402 



BOTANY 



PART II 



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. 340 A) or float free. The thallus of the marine genus Ulva (Ulva lactuca, 

 SEA LETTUCE) has the form of a large, leaf-like cell surface, and is two layers 

 of cells thick (Fig. 81, young plant). In Enteromorpha (Fig. 339) 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 appear red 

 on account of the haematochrome they contain and possess a violet-like odour. 



The cells have always only one nucleus 

 and also a single chloroplast. \ 



The asexual reproduction is accom- 

 plished by the formation of ciliated swarm- 

 spores. Sexual reproduction is effected 

 either by the fusion of planogametes, or 



FIG. 337. A, Scenedesmus acutus. B, The same, 

 undergoing division. C, Scenedesmus caudatus. 

 (x 1000. After SENN.) 



Fio. 338. Pediastrum granulatum. A, An old 

 cell-family : a, cells containing spores ; b, 

 spores in process of extrusion (the other 

 cells have already discharged their spores). 

 B, Cell-family shortly after extrusion of the 

 spores. C, Cell-family 4i hours later, (x 300. 

 After AL. BRAUN.) 



the sexual cells are differentiated as non-motile egg-cells and motile 'sperm atozoids. 

 Ulothrix zonata ( 2 ) (Fig. 340 A) is one of the commonest filamentous Algae. 

 The filaments of Ulothrix exhibit no pronounced apical growth ; they are 

 unbranched, attached by a rhizoid cell, and consist of 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 singly or 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 have only two cilia (JE). 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. 

 Ulothrix is dioecious, for gametes derived from the same filament do not fuse, but 

 only those of distinct origin. After a shorter or longer period of rest the zygotes are 



