202 K r i t s eil . Studies ou Cyanophyceae. 



1)6 duo to .sucli cliaiiges. AMieii a lilamoiit lias a low of liete- 

 rocysts one boliind tlie other (cf. F ritsch 04, p. 89) the distal 

 one has iio niucilagc envelope whatsoevor; the next heterocyst 

 is snrroundod by a layer of" miicilage, formed before its trans- 

 fonnatioii, the third has two sueli Layers, and so on — The 

 cell-sheath, as in tlie case of tlic tilaments treated witli lodiiK^, 

 beeomes better defined, when stained witli Vesuvin; the inner 

 investment llo^vever remains practically nncoloured. In youiig 

 rilanients. in wliich a cell-sheath is not yet recognisable, staining 

 with ^'esuvin makes the inner investment (then the only one ?) 

 particiilarly prominent; for the protoplast is separated from the 

 brownly stained external mucilage by a narrow coloiirless area, 

 representing the inner invostmont. 



(b) Nostoc. It is scarcely nocessary to give many details 

 here, as the strneture practically agrees with that of ÄnahaeiHi. 

 I was not able to obtain sporogenous lilaments and therefore 

 the recognition of many of the points Avas of considerable dif- 

 hculty. With the help of lodine or Yesuvin however the cell- 

 sheath was brought out prominently and especially the lateral 

 parts are then well defined. There can be no cloubt in such 

 stained specimens, that the lateral portions of the outer envelope 

 (cell-sheath) are only proper to the individual cells (cf. fig. 3b). 

 In many cases too the thick dark Uno, which marks the lateral 

 portion of the cell-sheath extends round on to the terminal 

 portion of the cell for a little way, which is probably due to a 

 slight thickeiiing of the margin of the open end of the cylin- 

 drical slieath ; the cell-sheath then appears | | in optica! 

 section. 



(c) Ghoocapsa and GloeotJ/ece. An interesting case is fur- 

 nished by Ghcocap-sa: in most cases (especially in the large- 

 celled forms) two eiwelopes are quite readily distinguishable 

 around the cell-contents viz. the colourless inner investment 

 (fig. 9, i. i.) and surrounding that a well-marked cell-sheath 

 (fig. 9. c. s), which here extends right round the cell. Division 

 takes jDlace in a manner quite similar to that described above 

 for Anahaeua: the cell beeomes constricted at its middle, whilst 

 the separating colourless mass (transverse wall) gradually appears 

 (fig. 9). Ultimately however it develops to a far greater extent 

 than in the j)revious cases, so that the daughter-cells become 

 more or less widely separated and the open cell-sheath gradu- 

 ally closes in right round each daughter-cell. AVe thus see that 

 the normal vegetative condition in a Glococapsa or GloeotJioce 

 presents the same structm'e as do the spores of an Anabaena, 

 i. e. in its reproductive cells this latter genus reverts to the 

 primitive type of structure, which probabiy appertained to its 

 an(;estors. The fact that the spores of an Aitabaena divide so 

 as to form a filament is due in part to a condensation of the 

 intercellular septum. in part to the loose diffluent character of 

 the external sheath (cf. p. -15, 4(^). 



