42 FIRST GROUP. THALLOPHYTES. 



directly, but swarm-spores which develope each into a new Stigeoclonium. Zoospores 

 as asexual organs of reproduction are found in all the better known members of the 

 group, though in some (Sphaeroplea) they only occur in the germination of the 

 oospores. 



The mode of sexual reproduction is not yet known in all the Confervoideae. In 

 some it consists in the conjugation of motile cells of similar form (gametes'), in other 

 words there is an isogamous union of gametes; in others the gametes are distin- 

 guished into a smaller male motile-cell or spermatozoid, and into a larger non- 

 motile female cell, the oosphere, which waits in a cell of the filament that produced 

 it for the impregnation which makes it an oospore. Both modes of fertilisation may 

 occur within one and the same subdivision ; in the Ulothricaceae, for example, the 

 genus Ulothrix is isogamous, Cylindrocapsa is oogamous. 



A larger number of fresh-water Algae, and among them species the most 

 widely distributed and most striking in appearance, belong to this group. They are 

 evidently nearly related to the Siphoneae on one hand and to the Conjugatae on the 

 other. The group called by Schmitz Siphonocladiaceae, to which the common 

 Alga of fresh and salt water, Cladophora, belongs, is intermediate between the Con- 

 fervoideae and the Siphoneae ; they resemble the latter in the character of the 

 protoplasm with its many small nuclei, but their thallus is multicellular. The power 

 possessed by the thallus of breaking up into isolated Protococcus-like cells and 

 of vegetating in this condition recalls the Protococcaceae, of which Hydrodictyon, 

 for example, has the like cell-structure to Cladophora. The resemblance to the 

 Conjugatae is rather one of habit ; these Conjugatae, as has been already said, 

 are a rather peculiar section of the Algae, though not quite so peculiar as the 

 Characeae. 



Certain families will now be described as examples of the course of development 

 in the Chlorosporeae. 



a. ULOTHRICACEAE. The thallus in this family consists of unbranched cell-filaments ; 

 each cell contains a chlorophyll-band running round the cell. The genus Ulothrix^ has 

 isogamous, Cylindrocapsa oogamous fertilisation. 



Ulothrix has two kinds of swarm-spores ; small ones with two cilia (inicrozoospores, 

 which are gametes), and larger asexual ones with four cilia, the macrozoospores. The 

 latter are formed singly, or two or four together, from the protoplasm of a cell of a fila- 

 ment ; they escape through a hole in the cell-wall, enclosed in a vesicle which is the in- 

 nermost layer of the wall of the mother-cell. Four zoospores, for instance, are formed 

 by successive bipartition of the protoplasm of a cell, the central vacuole remaining 

 behind as a hyaline bladder, as in many other similar cases. The zoospores are set free 

 by absorption of the enveloping membrane ; they are pear-shaped bodies, with four cilia 

 at the colourless pointed extremity which in motion is the forward end, and with a red 

 spot, the so-called eye-spot, on the side. The zoospores, when come to rest, fix them- 

 selves by their colourless extremity, secrete a cell-wall, and grow into a new Ulothrix- 

 filament. The sexual zoospores, the gametes, are all of the same shape and size, and 

 have only two cilia. They conjugate in pairs, the cilia being first of all entangled 

 together ; then they place themselves close to and finally unite with one another, the 

 process beginning with the hyaline pointed ends. The zygospore thus formed moves 



1 Doclel-Port, Die Kraushaar - Alge, Ulothrix zonata u. ihre geschlechtliche Fortpflanzung 

 (Pringsheim's Jahrb. f. wiss. Bot. X. p. 142). 



