360 



BOTANY 



PART II 



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

 a many-meshed net. 



Order 3. Ulotrichales. — The Ulotrichales exliibit, as compared witli 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 attaclied by a colourless basal cell to tlie 

 substratum (Fig. 281, J) or float free. The thallus of the marine genus Ulva {Viva 

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

 layers of cells thick (Fig. 5, young plant). In Entcrommylia (Fig. 280) the thallus 

 is ribbon-shaped, either cylindrical or flattened ; when youug 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, 



dB m on leaves. To this family belongs 



^H ^fl Trente'pohlia {ov Chroolepus) Jolithus, 



^^B ^M often found growing on stones in 



^^ ^m mountainous regions. The cell fila- 



ments 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 

 accomplished by the formation of 

 ciliated swarm-spores. Sexual re- 

 production is effected eitlier by the 

 fusion of planogametes, or the sexual 

 cells are differentiated as non-motile 

 egg-cells and motile spermatozoids. 



Ulothrix zonata {^^) (Fig. 281, A) 

 is one of the commonest filamentous 

 Algae. The filaments of Ulothrix 

 exhibit no pronounced ai)ical growth ; 

 they are unbranched, attached ])y a rhizoid cell, and consist of rows of short 

 cells ; each cell contains a band-shaped chloroplast. The asexmil 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-sjiores escape through a latei'al 

 opening (JB) 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 otlier 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 tlieir 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 tlie zygotes are 

 converted into unicellular germ plants {J), 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. 



Fio. 'ifiO.—Enteromorpha compressa. (I nat. size.) 



