CHAPTER XV 



POND-SCUMS, SEAWEEDS, ETC. 



THE organisms considered in the last chapter are simple 

 representatives of a lowly group of the Vegetable Kingdom 

 termed the Alga, to which also belong the Pond-scums and the 

 Seaweeds. The Algje, with very few exceptions, possess 

 chlorophyll and feed in the same way as other green plants. 

 They are distinguished mainly by the relatively simple con- 

 struction of their body, which varies from a single cell to a 

 multicellular thallus of some complexity, and by the simple 

 character of their reproductive llrgans. 



The simplest type of multicellular thallus is a filament or 

 row of cylindrical cells, all nearly identical, both in structure 

 and function. Examples are furnished by Ulothrix (Fig. 115, c), 

 Spirogyra, and (Edogonium (Fig. 117, h), genera whose species 

 commonly occur as floating tangled masses in ponds and stagnant 

 ditches, although some are found in flowing water. In most 

 such forms every cell is capable of division, but in (Edogonium 

 this is restricted to occasional cells, recognisable by the presence, 

 at one end of the cell, of a succession of fine rings (known as 

 caps), formed singly at each division (Fig. 117, h, c.). 



The habit of the plant becomes slightly more complex when 

 the filaments are branched, as in Cladophora (Fig. 104, A) 

 and Ectocarpus (Fig. 119, /). Species of the latter commonly 

 occur as brown tufts or tresses attached to diverse substrata 

 in the rock-pools on the seashore, whilst those of the former, 

 though also found in similar situations, are commoner in well 

 aerated fresh-water. Each branch of a Cladophora terminates 

 in a pointed apical cell, with specially dense green contents. 

 These cells are the growing points, by whose enlargement and 

 division the branches gradually lengthen, the segments cut off 



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