432 



PROTOPHYTA 



intricately interwoven, owing to the heterocysts maintaining their position 

 when once formed, while the portions of the filament between them 

 continue to lengthen by cell-division. 



Most of the Nostocaceae inhabit fresh, usually stagnant, water, bog- 

 pools, &c. ; a very few grow in salt water. Several species of Anabcena 

 Bory, Aphanizomenon Morr., and some other genera, frequently collect 

 as a bright bluish-green scum on the surface of bog-pools ; Aphanizo- 

 menon flos-aquae (Rlfs.) is one of the organisms which contribute to the 

 phenomenon know as the ' breaking of the meres.' The 

 decay of species of Anabaena has a large share in causing 

 the foul odour and injurious properties of stagnant water. 

 Of the numerous species of Nostoc Vauch., a few 

 only swim freely in water ; these are usually minute with 



a colourless * thallus ' more 

 or less nearly spherical and 

 distinctly lamellated, with 

 a spontaneous power of 

 active motion in the water. 

 The larger number are 

 terrestrial, growing on wet 

 rocks, among moss on 

 damp soil, or even on the 

 comparatively dry ground, 

 forming conspicuous jelly- 

 like masses, often as much 

 as from one to two inches 

 in diameter, usually of a 

 green, yellow, brown, or 

 sometimes red colour, the 

 'frond' enclosing a large 

 number of bright blue- 

 green filaments (fig. 358). It is not uncommon for other chlorophyl- 

 lous protophytes or algae, the mycele of fungi, and the protoneme 

 of mosses, to become enclosed, and even to continue growing, within 

 their gelatinous envelope. Species of Nostoc and Anabaena are found 

 in hot springs, and several species of the same genera are frequently 

 to be met with within the cells of living aquatic or land plants, where 

 they carry on an epiphytic existence (see pp. 28, 165, 171). They 

 occur with especial frequency in Lemna, causing yellow spots in the 

 leaves. 



Isocystis Bzi. is probably the simplest form of the Nostocaceae. It 

 consists of delicate moniliform filaments, which do not form colonies, but 



FIG. 361. Aphani- 

 zomenonJJos-aqucE 

 Morr., with spore 

 ( x 400). (After 

 Cooke.) 



FIG. 362. Cylindrospermnm macro- 

 spermutn Ktz., with heterocysts 

 and spores ( x 400). (After Cooke.) 



