Ulotrichacece 73 



Family 3. ULOTRICHACEJE. 



This family includes a few genera which are readily distin- 

 guished from other plants of the Chsetophorales by their un- 

 branched habit and by the structure of their cells. The thallus 

 is a simple filament, consisting of cylindrical or doliform cells, as 

 in Ulothrix, or of rounded cells arranged in a single series and 

 enveloped in a thick mucous coat, as in Hormospora and Radio- 

 jilum. The cell-wall is always hyaline and colourless, but varies 

 much in thickness. It is sometimes delicate, sometimes thick and 

 lamellose, and sometimes the outer layers are diffluent. There is 

 a single, parietal, plate-like chloroplast in each cell, with an entire 

 or variously lobed margin, and containing one or many pyrenoids. 

 A single nucleus is present in the cytoplasm. 



Asexual reproduction takes place in several ways. Sometimes 

 aplanospores are produced (fig. 20 Da; fig. 21 Fa), or numbers 

 of akinetes 1 are formed by the enlargement of certain cells and the 

 gelatinization of the outer portions of their original cell-walls 

 (fig. 21 E and I) ; these may be resting-spores (hypnospores) or 

 they may germinate directly. Sometimes the thallus is multiplied 

 by a general dismemberment of the filament into single cells or 

 groups of cells, each cell or group developing into a new filament. 

 Zoogonidia of two kinds are produced, often from different cells of 

 the same filament; small microzoogonidia with two cilia and 

 larger macrozoogonidia with four cilia. The microzoogonidia are 

 produced from certain of the. vegetative cells which have become 

 microzoogonidangia and in the larger species of Ulothrix, such as 

 U. zondta, 16 or 32 are produced from each gonidangium, but in 

 U. subtilis only 2 or 4 are produced. Similarly 2, 4, or 8 macro- 

 zoogonidia are usually produced from a macrozoogonidangium, but 

 in U. subtilis only one arises. It is occasionally observed that the 

 entire contents of the cell are not used up in the formation of the 

 macrozoogonidia (vide fig. 21 G). The zoogonidia germinate di- 

 rectly on coming to rest, sometimes even within the mother-cell, 

 and the plants which arise by the germination of the macro- 

 zoogonidia are larger than those which arise from the micro- 

 zoogonidia. This accounts for the variability in size of the 

 filaments which is so often observed in a collection of any one 



1 This was first shown by Wille in Bot. Centralbl. xi, 1882, p. 113. 



