Resting -spores 



113 



into normal frustules. Similar divisions of the protoplast were observed in 

 Chxtoceras borealis. This sporulation is not essentially different from the 

 microspore-formation described by Gran in Chtrioceras decipiens and by 

 Schiller in Chxtoceras Lorenzianum. Murray also observed the formation 

 of one or two smaller frustules within the mother-cell both in Coscinodiscus 

 concinnus and in Biddulphia mobiliensis. These observations are interesting 

 if only because they indicate a possible means of rapid multiplication of 

 plankton-diatoms, but further comment is not possible until the matter has 

 been re-investigated. 



RESTING-SPORES. In a few species of plankton-diatoms thick-walled 

 resting-spores have been found. They 

 occur in at least one marine species 

 of the genus Rhizosolenia, and in the 

 two freshwater species, R. setigera 

 and R. morsa (W. & G. S. W., '09). 

 They are also known in Ch&toceras, 

 Bacteriastrum and Attheya. In 

 these genera only one resting-spore 

 is formed within the mother-cell. 

 The protoplast shrinks until it 

 occupies only a small part of the 

 original cell, after which it surrounds 

 itself with a thick siliceous wall, 

 which often develops spines or pro- 

 cesses. In Melosira italica, 0. Mu'ller 

 has observed the formation of pairs 

 of resting-spores. Resting-spores 

 of this nature appear to be formed 

 at the end of the vegetative season, 

 and up to the present time their 

 germination has not been observed. 

 Resting-spores have also been ob- 

 served in Surirella spiralis (G. S. W., 

 '12), a diatom which occurs in boggy 

 marshes. In this case eight thick- 

 walled spores were formed within 

 the mother-cell (fig. 83). 



In some diatoms a resting con- 

 dition has been noticed in which 

 the protoplast has shrunk in size 

 and a pair of new valves has been 



w. A. 



Fig. 82. A, Part of cell of Rhizosolenia morsa 

 W. & G. S. West with resting-spore. B, 

 Resting-spore of Chtetoeeras ceratosporum 

 Ostenf. within the old mother-cell-wall. D, 

 Similar state of Ch. gracile Schiitt. C, Part 

 of filament of Ch. paradoxum Schiitt showing 

 stages a d in formation of resting-spores ; 

 c is a fully-formed resting-spore in surface 

 view ; d, a similar fully-grown spore in optical 

 section. (B and D, after Ostenfeld ; C', after 

 Schiitt.) 



