438 



LECTURE XXVII. 



they cut more or less exactly at right angles, and as they become further distant 

 from one another in this course towards the margin new lines of cell-walls are 

 continually being intercalated between them. The fan-shaped structure thus 

 produced is shown particularly clearly on the right side of the figure; and it is 

 noticed that the very various curvings of the radiating fan-like walls stand in 

 definite relation to the curving of the line bounding the circumference which is, 

 as a rule, cut by them at right angles. 



Besides this, however, the figure shows, especially on the left side, a larger 

 number of lines — i.e. cell-walls — which run parallel with the periphery, or, to put 

 the fact better, conformably with it, and in the main these lines cut the previous 

 system of fan-like walls at right angles. Geometrically expressed, the two systems of 

 lines which divide up the surface of the body of the Alga into areolae or cells of 

 nearly equal size are orthogonal trajectories. The regular pattern of this cellular 

 structure comes into existence essentially from the fact that only the cells at the 



Fig. 2Ti.—Melobcsia Lejoltsii {an Alga of the subdivision Floride«) seen from the 

 upper surface (after Rosanofl). 



external margin of the surface grow radially outwards, and become divided now and 

 then in the tangential direction. Cell-divisions thus take place only at the margin, 

 and the cells further distant from it grow no more, either in the radial or in the 

 tangential direction. Were the latter the case, the cell-network could only maintain 

 its form by a perfectly equal distribution of the growth : in every other case, however, 

 it must be altered. 



Attention may here be drawn by the way to the further fact that the fan-like 

 radiating walls have all been drawn, even on the right side of the figure, though in 

 this portion the majority of the walls running tangentially have been omitted. It is 

 seen that in this way there arises a cell-network different from that on the left side ; 

 though the difference only depends on a minor point, viz. that the tangential 

 division-walls exist in smaller numbers than on the left side, and we could easily 

 quote examples of other Algse in which the whole cell-network resembles that 

 represented on the right side of the figure. In such a case, however, the body of 



