SCH1ZOPHYTA. 33 



portion of a filament are invested with a thick cell-wall and increase in size, their 

 protoplasm becoming at the same time dense and of a yellowish-green colour. The 

 exosporium bursts in germination, and each spore produces a new nostoc-thread which 

 soon invests itself with an envelope of mucilage. In some cases, as in Nostoc Line/da, 

 the young plant, instead of growing directly into a nostoc-thread as usual, changes its 

 colour to yellow and becomes a hormogonium, which after passing through a period of 

 rest again assumes the ordinary bluish green colour and developes into a nostoc- 

 colony. 



Besides the genus Nostoc just described there are the following subdivisions of the 

 Nostocaceae. 



. The OSCILLATORIEAE. These are rigid cylindrical filaments of varying thickness, 

 often extremely slender, and divided by very delicate transverse walls into disc-like 

 cells. The cells are all alike, there being no heterocysts. The filaments are not 

 straight, but somewhat twisted into a very oblique spiral ; they turn on their axis, and 

 when growing together in large numbers (in water or on moist ground) become matted 

 into balls or pellicles of a dark green colour ; a bunch o.f these filaments placed in water 

 or on wet paper assumes a star-like arrangement in consequence of these- movements, 

 as Nageli has shown l . 



b. The SCYTONEMEAE form branched filaments, consisting at least in their older 

 portions of several rows of cells, and enclosed in thick gelatinous envelopes. Hetero- 

 cysts, hormogonia and spores occur. To this subdivision belong Scytonema, Sirosiphon, 

 and others. The mode of branching is peculiar ; any cell of a filament thrusts itself 

 past the cell that stands above it, and grows out into a branch. 



c. A distinct section of the Nostocaceae comprises those forms, in which the filaments 

 end in a hair ; the cells toward the extremity of the filament become narrower and lose 

 their protoplasm. The heterocysts are at the opposite end of the filament, which 

 growing gradually thinner from above downwards takes the form of a riding-whip with 

 a knob (the heterocyst) at the top. The branching is the same as in Scytonema. To 

 this subdivision belong the RIVULARIEAE especially, together with some marine species. 

 The Rivularieae form soft greenish-brown lumps of jelly which are found free floating 

 or attached in calcareous waters. In the first case they are spherical in shape, in the 

 second hemispherical, the smallest about half a millimetre in diameter, the largest the 

 size of a hazel-nut. A large number of moniliform filaments with roundish cells lie 

 radially disposed in the jelly. The filament increases in length by transverse division 

 of its cells. The cell that lies immediately over the basilar heterocyst becomes a spore, 

 and grows in breadth and especially in length ; its cell-contents become more dense, and 

 it invests itself with a firm cell- wall. Ultimately the whole colony is broken up and the 

 spores alone remain. These after a time germinate, dividing each into four to twelve 

 shorter cylindrical pieces, each of which divides repeatedly till upwards of one hundred 

 cells have been formed ; the cells then become rounded off and give the filament a 

 moniliform appearance. The elongation of the filament bursts the wall of the spore, the 

 upper end of the filament emerges, and the lower portion ultimately slips out of the 

 sheath. The cells at the extremities of the thread become pointed, and finally the whole 

 thread breaks up into several pieces, which crowd together into a bundle or tuft. Each 

 portion of the filament now becomes elongated at one end into a segmented hair, while 

 the cell at the other end forms the heterocyst. This tuft of filaments, the product of 

 a germinating cell, represents once more a young rivularia-family, whose filaments 

 are already enclosed in a jelly produced by the swelling up of the cell-walls. The 

 multiplication of the filaments of a growing family is effected by 'apparent' branching 

 (as in Scyfonema), that is, one of the lower cells becomes a new heterocyst, while the 

 portion of the filament between it and the old basilar cell grows into a perfect thread 

 and takes up a position alongside of the mother-filament. 



1 [Hansgirg, Bemerk. iiber die Bewegungen d. Oscillarien (Bot. Zeit. 1883).] 



