io THALLOPHYTA 



south-western mountains. It has the capacity of growing in a drier situation 

 than most algae, the thallus being aerial. The plant consists of dense tufts of 

 branched filaments of a bright colour. A few cells of a filament are shown 



separately in Plate 2, Fig. N 2. Reproduction is by means of zoospores, 



produced in nearly spherical sporangia, which are borne here and there, either 

 terminally or laterally, on the filaments. 



Two other green algae may be referred to, one fresh-water, the other 

 marine, as remarkable instances of peculiar development of cells. 



The fresh- water alga, called the " water net " {JlyJrod'ictyon rct'iculiitum\ 

 is found in vleis in the spring. A small part of one of these nets is shown, 

 natural size, in Plate 2, Fig. I. Each of the green, nearly spherical segments 

 of the net, consists of a single cell*. The most usual method of repro- 

 duction is by the formation of large numbers or zoospores within some of 

 the old cells of the net. These swarm about within the mother-cell and 

 then come to rest with their ends adjoining in such a manner as to form 

 a new net within the cell. On the subsequent rupture of the mother-cell, 

 the young net is set free, and, by growth of the cells, gradually attains its 

 full size. 



There is also a somewhat complex method of sexual reproduction, which 

 is less commonly met with, and need not be considered here. 



The marine example is Codium tomentosum, a dark green sea-weed with 

 a soft cylindrical and repeatedly forked thallus several inches in length. The 

 remarkable feature of this plant is that the whole individual is made up of 

 a single much-branched tube, which is developed from a single cell ; the ends 

 of all the branches enlarge and become club-shaped, and being closely packed 

 together at the surface of the thallus they form a kind of pseudo-cortex for 

 the plant. One of these club-shaped tips is shown, considerably magnified, 

 in Plate 2, Fig. G. Growing out from it, lateral!}', is one of the reproductive 

 organs {garnet angiwwi). In this a large number of minute, motile sexual 

 cells (gametes) are produced, which fuse together in pairs, the resulting zygotes 

 growing out to form new plants. 



VI. CONJUGATAE. 



Quite a number of these are found in our streams, vleis and pools; the) 

 consist of unbranched filaments, each filament being made up of a single row 

 of cells, all exactly alike. Among the commonest of these are species of the 

 genus Spirogyra (Fig. 5). Each cell of Spirugxm is bounded by a cell-wall, 

 and contains a central nucleus and a lining layer of protoplasm. Embedded 

 in the latter are one or more conspicuous, spirally wound chloroplasts, from 

 which the plant takes its name. 



The plant has two methods of reproduction. One is by the repeated 

 division and growth of the cells, accompanied by the breaking up of the 



* More accurately a CO&lWCyte (/'.< '. a multinucleate " cell "). The same remark 

 applies to the cell of Codium. 



