430 PROTOPHYTA 



Order i. — Nostocace-€. 



The Nostocacese are distinguished from all the other families of fila- 

 mentous Cyanophyceae by the less close connection with one another of 

 the pseudocysts of which the filament is composed, giving it always more 

 or less of a moniliform or necklace-like appearance. In most of the 

 genera these pseudocysts are spherical or elliptical ; but in Nodularia 

 (Mert.) they are disc-shaped, and more closely connected with one 

 another than in the other genera. The extent to which the filaments 

 are enveloped in mucilage varies greatly. In some genera (Nodularia) 

 each filament is enclosed in a distinct hyaline sheath. In others 

 (Anabsena, Bory, Aphanizomenon, Morr., Sphaerozyga, Ag., Cylindro- 

 spermum, Rlfs.) this sheath is obscure or wanting ; in Aulosira (Kirch.) 

 the filaments are either naked, or enclosed in a dry membranous sheath. 

 In most species of Nostoc (Vauch.) a single filament or a number of 

 filaments are enclosed in a hyaline jelly, often of considerable size and 

 definite outline, formed from the more or less complete coalescence of 



the separate sheaths, which can 

 still sometimes • be indistinctly 

 made out. Such a jelly-like mass, 

 which sometimes floats freely in 

 the water of bog-pools, but is 

 more often found on damp soil, is 

 sometimes called a 'thallus' or 



Fig. 3s8. — Nostoc commune L. (natui^ size). £ r j j t^ ■ • /- 



(After Cooke.) iroud. It may vary m size from 



0-2 mm. to that of a small plum ; 



is often of a green, violet, or blue colour, and, in the larger terrestrial 



species, the outer layers of the integument are more or less hardened, 



forming what is known as a ' periderm.' 



The Nostocaceas display no differentiation of the two extremities of 

 filament. In nearly all the genera some of the cells are here and there 

 replaced by heterocysts — cells incapable of further division, of a slightly 

 larger diameter, and with a somewhat thicker cell-wall, which is often 

 yellow, the green protoplasmic endochrome being replaced by a watery 

 colourless cell-sap. These heterocysts, the function of which is unknown, 

 may be terminal, basal, or intercalary. Others of the cells, always in this 

 case intercalary, are, in most species, replaced by resting-spores, also dis- 

 tinguished from the ordinary cells by their larger size and thicker cell- 

 wall, but containing a green endochrome. They are often formed more 

 or less in connection with the heterocysts. In Cylindrospermum the 

 terminal cells of the filaments become heterocysts, and the spores, some- 



