Vaucheriacese 



245 



pairs, and embedded in the peripheral protoplasm under each pair of cilia is 

 a small nucleus (fig. 158 (7). The central part of the zoogonidium is occupied 

 by a large sap vacuole traversed by delicate strands of protoplasm. In the 

 outer zone of protoplasm, but within the layer of nuclei, are numerous small 

 chloroplasts which give the zoogonidium a deep green colour. As first 

 suggested bySchmitz (79) this large structure can be regarded as compound 

 in character, being constituted of an aggregate of numerous small biciliated 

 zoogonidia. The disposition of the cilia in pairs, each pair being related to 

 an underlying nucleus, certainly supports this suggestion, as does the fact 

 that the whole of the contents of the gonidangium go to form a single motile 

 gonidium with many paired cilia. 



The zoogonidia generally escape in the morning, that is to say, after the 

 plants have been in darkness for some time. They move but slowly and 

 rarely continue active for more than twenty minutes. On coming to rest 

 the cilia are at once withdrawn and a cell -wall is developed. Klebs ('96) 



-~ 71 



Fig. 158. Vauclieria sessilis (Vauch.) DC. A, zoogonidangium ; B, escaped zoogonidium; 

 C, somewhat diagrammatic representation of a zoogonidium, upper two-thirds in surface 

 view, lower third in section ; ch, chloroplasts ; n, nuclei ; v, central vacuole. A and B, 

 x 160 ; C, x 220. 



states that zoogonidia can always be produced when filaments which have 

 been kept moist for some days are soaked with water, or when they are 

 removed from running into still water, or when they are transferred from a 

 dilute nutritive solution into pure water, or from light to darkness. After 

 coming to rest the zoogonidia germinate almost immediately by the protrusion 

 of one or more tube-like filaments (generally two at opposite poles), one at least 

 of which attaches itself to the substratum by a colourless branched rhizoid. 



Solitary aplanospores are sometimes produced in aplanosporangia, as in 

 Vaucheria geminata, and their formation may be induced by cultivation in 

 air which is only slightly damp. They are, however, only rarely observed 

 under natural conditions. 



Under some circumstances, such as when the plants become almost dried 



