PROTOPHVTA. 



247 



Fig. 165.— Mode of cell-division ; 

 GlceoMpsa, 



(Fig. 165), which divide in all directions, the latter imbedded in a stratified jelly; 

 GlcEothece, imbedded in a stratified jelly but dividing in one direction only; and 

 Merismopedia, the cells of which divide cross-wise in a single plane. 



2. The NostoeaeeaB form lumps of mucilage or gelatinous pellicles which float 

 in water or lie on damp earth or among moss. In the jelly are serpentine moniliform 

 rows of roundish cells, a few larger cells, termed Hetero- 



cyjts, the contents of which are of a different colour, being 

 interposed at intervals. The filaments increase in length 

 by the division of the individual cells, thus constantly 

 adding to the coils which lie in the jelly that they ex- 

 crete. New colonies are, according to Thuret^, formed 

 in the following manner : — The jelly of the old colony 

 becomes softened by water, the portions of the threads 

 lying between the heterocysts become detached, separate 

 from the jelly, and straighten themselves, while the hetero- 

 cysts -themselves remain in the jelly. After they have 

 entered the water, the old portions of the threads become 

 endowed with motion like the Oscillatorieae, and their exit 

 is probably caused by this movement''^. The roundish 

 cells of the filaments grow transversely, i.e. at right 



angles to the axis of the filament, and then divide, the division-planes being parallel 

 to the axis of the filament, which now consists of a series of short articulate threads, 

 the axis of whose growth is at right angles to its own. The threads which are thus 

 formed increase in length and join, placing their terminal cells in contact [alternately 

 above and below in each successive thread], and thus unite into a single curved 

 A^oj^or-filament. Individual cells, apparently without any definite law, become hetero- 

 cysts. In the meantime the gelatinous envelope is developed, and the new colony, 

 which is at first microscopic, attains the size of a walnut^. 



3. The OscillatorieaB consist of rigid cylindrical filaments of various thickness, 

 often extremely slender, divided into disc-like cells by very delicate transverse septa. 

 The filaments are not straight, but somewhat coiled in the form of a very oblique 

 spiral ; they revolve on their axis, and become matted, when large numbers grow 

 together (in water or on moist earth), into balls or pellicles. When a lump is placed 

 in water or on wet paper, Nageli has shown that it assumes a star-like arrangement 

 in consequence of these movements. 



4. The Rivularieae * form soft greenish blue lumps of jelly which swim about in 

 stagnant water or grow attached ; in the first case they are spherical, in the second 

 hemispherical, the smallest about ^ millimetre in diameter, the largest the size of a 

 hazel-nut. 



^ Thuret, Observations sur la reproduction de quelques Nostochinees, Mem. de la soc, imp. 

 des sci, nat. de Cherbourg, vol. V. 1857. [Ann. and Mag. Nat. Hist. 1858, vol. II.] 



^ The^e motile threads of Nostoc were seen by Janczewski to enter the young stomata on the 

 lower side of the thallus of Anthoceros IcBvis, where they further develope into round balls. Such 

 colonies of Nostoc have been known for a long time in cavities and in the tissues of different Hepa- 

 licse {Blasia, Pellia, Diplolcena, Aneura, Riccia), but have generally been considered endogenous 

 gemmx of these species, until Janczewski proved their true nature. Nosioc also estabhshes itself in 

 the large porous cells of the leaves of Sphagnum. The entrance of Nostoc into the parenchyma 

 of the stem of a dicotyledonous plant, Gunnera, is brought about, according to Reinke, in a different 

 manner ; the deeper-lying parenchymatous cells of the outer part of the stem, themselves covered by 

 layers of parenchyma, are densely filled v/ith colonies of the Alga. (Bot. Zeitg. 1872, pp. 59 

 and 74.) [See also Ann. des Sc. Nat. 1872, p. 306, and Quart. Journ. Micr. Sc. 1873, p. 369.] 



^ [Archer has described the occurrence of 'spores' in Nostoc paludosum which were always 

 placed singly between the heterocysts. Quart. Journ, Micr. Sc. 1872, p. 367.] 



* De Bary, Flora, 1863, P* 653. 



