The Freshwater Algae of Africa. 511 



pointed ends, and a membrane which, in older cells, shows two distinct 

 layers and slight thickening at the poles, although at first uniformly 

 thin. Such cells give rise to four-celled stages by division of the 

 contents into four at right angles to the longitudinal axis of the 

 mother-cell (Fig. 8, £). At first the membrane of the latter preserves 

 its elliptical outline and can easily be traced right round; the polar 

 thickenings are also very plain in such stages (Fig. 8, i). Later, 

 however, as the daughter-cells increase in size, the membrane becomes 

 bulged out opposite the cells and appears invaginated between them, 

 and it is now not always easy to trace it all the way round (Fig. 8, 

 <I and e). The original polar thickenings are always more or less 

 obvious, and are also plainly seen in end-views (Fig. 8, b) ; the latter 

 show the cells to be spherical in section. After reaching a certain 

 size the daughter-cells in their turn divide ( always in a plane at right 

 angles to that of the mother-cells). By a repetition of this process 

 large plate-like colonies gradually arise, like those shown in Fig. 8, 

 a and g. The membrane of the original mother-cell is even recognisable 

 in such late stages (Fig. 8, a,f, g), but the walls of the successive 

 daughter-cells are not by any means so plainly evident, and appear to 

 undergo gelatinisation at an early stage. In many cases all the 

 daughter-cells lie in the same plane (Fig. 8, a and g), but colonies 

 are not rarely to be found in which some of the secondary groups lie 

 in a plane perpendicular to that of the others (Fig. 8, f). This 

 appears to be due to a shifting of one or more of the products of 

 a certain stage of the division-process through a right angle, the 

 actual division, as far as I have seen, always taking place in a plane 

 at right angles to that of the momentary mother-cell. As the figures 

 show, one or more of the cells of a given group not uncommonly fail 

 to divide (Fig. 8, a,f, g). 



Fig. 8. — Scenedesmus cohaevens, n. sp. a. Large mature colony, in which 

 only three of the original cells of the four-celled mother-colony have 

 divided (the undivided one is on the left) ; in the second group from 

 the right the uppermost cell has again failed to divide. //. Four-celled 

 colony, seen from the end. showing the investing membrane of the 

 original mother-cell. c. Single cell, d and e. Mature four-celled 

 colonies, in which the investing membrane of the mother-cell is plainly 

 visible, as well as the lateral (originally polar) thickenings ; in e the 

 right-hand cell has escaped from the surrounding membrane /'. A 

 young colony, produced from a four-celled one by division of three 

 of the cells, two of these having shifted through a. right angle, so 

 tli at the four-celled groups produced by them are in a plane perpen- 

 dicular to that of the original colony. ;/. A large mature colony, show- 

 ing various phases of division. //. Diagram to explain the formation 

 of the large plate-like colonies; I-IV, 1-4, and "-</ various steps in 

 the process of subdivision. J. A young four-celled colony in which the 

 mother-cell membrane is no1 as vol distended by the growth of the 

 daughter-cells. A.11 figures except h x 750. 



