ALGJE 1 85 



closed in a more or less strongly developed gelatinous sheath. The 

 greater number of families exhibit both a sexual and a non-sexual mode 

 of reproduction, though in some cases one or the other mode has not 

 yet been detected. In the great majority of families the non-sexual 

 propagating bodies are motile cells or zoospores, minute masses of proto- 

 plasm formed singly from the whole contents of a cell by rejuvenescence, 

 or more often in large numbers by free-cell formation, destitute of a true 

 cellulose membrane, but containing protoplasm and a contractile 

 vacuole, and provided with two or sometimes a larger number (rarely 

 only one) of vibratile cilia, by means of which they move about actively 

 for a time, then come to rest, excrete a cellulose membrane, and de- 

 velop into a new plant. In one class only, the Florideae, are the zoo- 

 spores replaced by non-motile tetraspores ; in the Conjugatse and 

 Fucaceae they are altogether wanting. The simplest form of sexual 

 reproduction is that of conjugation, or the coalescence of two compara- 

 tively undifferentiated masses of protoplasm. These masses of proto- 

 plasm may be either the contents of stationary cells, which are nearly 

 or quite alike, as in the Conjugatae, or they may be motile ciliated 

 bodies indistinguishable from zoospores — zoogametes — or they may be 

 distinguished from the true zoospores by their smaller size. From 

 the conjugation of zoogametes there is a gradual transition through 

 intermediate stages to a true sexual process, the impregnation of a 

 stationary oospkere by a motile antherozoid, usually much smaller than 

 the oosphere, the result being the production of an oosperm by the 

 encysting of the oosphere in a coat of cellulose. In the higher families 

 the oospheres and antherozoids are formed in special cells or organs, 

 known as oogones and antherids respectively. In the Florideae the 

 process displays very great complication ; the structure in which the 

 Oosphere is formed is known as the carpogone ; the fertilised oosphere is 

 the carposperm, which often breaks up into carpospores. In this class 

 also the antherozoids are replaced by motionless protoplasmic bodies 

 known a.s pollinoids. Multiplication by the simple fission of individuals, 

 by the detachment of gemma, or buds,' and by the encysting of special 

 cells or masses of cells into cysts, also occur. In the green Algae 

 {Confervoideae heterogamae and isogamae and Conjugatae) single non- 

 motile cells which become detached for the purpose of propagation are 

 termed by Wille akinetes when they are formed without rejuvenescence, 

 aplanospores when formed by rejuvenescence. The former occur in 

 Trentepohlia (Mart.), Conferva (L.), and Ulothrix (Ktz.), as well as in the 

 Nostocaceae and Rivulariaceae, the latter in the Confervaceae. The two 

 kinds pass into one another, and akinetes into vegetative cells, by in- 

 sensible gradations. 



