A SOLID AGGREGATE 201 



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 

 superficial 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 not only divide 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 : the largest and most complicated sea-weeds 

 are the great olive-brown forms known as " tangles ' : or 

 " kelp," so common at low water-mark. They belong to 

 various genera, of which the commonest British form is 

 Laminaria. 



Laminaria (Fig. 45, A) consists of a cylindrical stem, 

 which may be as much as two metres (6 ft.) in length and 

 5 or 6 cm. in diameter : its proximal end is fastened to the 

 rocks by a branched root-like structure, while distally it 

 expands into a great, flat, irregularly-cleft, leaf-like body, 



