198 THALLUS OF GREEN 



Division of labour is also well marked in Ectocarpus, where 

 the thallus usually exhibits a differentiation into upright and 

 prostrate portions, both of which are branched and filamentous, 

 the latter acting as the organ of attachment. The growth 

 of the erect portion takes place by the division of certain cells 

 near the ends of the branches, beyond which the latter usually 

 consist of almost colourless tapering cells (Fig. 119, b). 



An unusual type of thallus is that of Vaucheria, the species 

 of which form rather coarse dark green wefts on damp soil (e.g. 

 by streams or in greenhouses) or in pools of fresh or salt water. 

 The branched filaments, though of considerable width and 

 length, are uninterrupted by septa (Fig. 118, A), and might as a 

 consequence be regarded as consisting of single cells. Since, 

 however, each contains numerous nuclei (Fig. 118, D, n.), it is 

 better compared with a multicellular organism where too the 

 cytoplasm exhibits continuity (cf. p. 24), but where mechanical 

 support is afforded by the walls separating the uninucleate por- 

 tions. As a matter of fact the filaments of Vaucheria depend 

 for their rigidity entirely upon turgor, and very readily collapse. 

 Vaucheria displays a division of labour similar to that of Clado- 

 phora, inasmuch as it often possesses an attaching organ and 

 growth is localised at the tips of the branches. 



At low tide on rocky shores one often sees thin crinkled slimy 

 sheets of a vivid green colour ; these belong to the Alga Ulva 

 (the Sea Lettuce). The thallus, which may reach a diameter of 

 a foot or more, consists of two superposed layers of cells of a 

 uniform character throughout (Fig. 116, E, F) except where 

 they constitute the attaching base. Very young plants of Ulva, 

 like those of most higher Algae, begin as a simple unbranched 

 filament, whose cells, however, soon undergo division in several 

 directions to produce the flat thallus. 



Most of the genera hitherto studied are so-called Green Algae 

 (Chlorophycea), 1 the majority of which inhabit fresh-water. The 

 bulk of the Seaweeds, however, are brown or red owing to the 

 presence of special pigments in the chloroplasts side by side with 

 the chlorophyll. The colouring matters are readily extracted with 



1 To this group also belong the Stoneworts (Cham), which, however, 

 exhibit much greater elaboration in their vegetative and reproductive 

 structures than other Green Algae. 



