XX SOLID AGGREGATES 203 



at right angles to the long axis of the filament, so that growth 

 takes place in one dimension of space only, namely in length. 

 In Monostroma the plane of division may be inclined in any 

 direction provided it is perpendicular to the surface of the 

 plant, so that growth goes on in two dimensions of space, 

 namely in length and breadth. 



Another of the flat, leaf-like, green sea-weeds is the very 

 common genus Ulva^ sometimes called "sea-lettuce." It 

 consists of irregular, more or less lobed expansions with 

 crinkled edges, and under the microscope closely resembles 

 Monostroma, with one important difference : it is formed 

 not of one but of two layers of cells, and is therefore not a 

 sui:)erficial but a solid aggregate. To return to the geometrical 

 analogy used above it is to be compared not to a plane but 

 to a solid body. 



As in Monostroma growth takes place by the binary 

 fission of the cells. But these divide not only along variously 

 inclined planes at right angles to the surface of the plant 

 but also along a plane parallel to the surface, so that growth 

 takes place in all three dimensions of space — in length, 

 breadth, and thickness. 



Ulva may be looked upon as the simplest example of a 

 solid aggregate, being built up of similar cells, and therefore 

 exhibiting no cell-differentiation. 



We shall now make a detailed study of a solid aggregate 

 in which the constituent cells differ very considerably from 

 one another in form and size, the result being a degree of 

 complexity far beyond anything we have hitherto met with. 



Nitella (Fig. 44, a) is a not uncommon fresh-water weed, 

 found in ponds and water-races, and distinguished at once 



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