THE ALGAE 107 



as we shall see later, are concerned in the formation of Lichens. Their wide 

 distribution, coupled wdth their peculiar structure, seems to indicate that 

 they are probably the most ancient and primitive of the Algae. 



The Cyanophyceae are divided into several orders of which we shall 

 consider only the Hormogonales, which includes most of the filamentous types. 



Hormogonales 



The Hormogonales are Cyanophyceae in which the filaments have no 

 individual membrane on the vegetative cells. The trichomes are simple, 

 free and sometimes coiled. They are septate, but the septa are frequently 

 obscure. The cells are very uniform, short and discoid or rectangular in 

 shape, with a uniform granular cytoplasm, divisible into two regions, 

 a peripheral one containing the pigment together with glycogen and oil- 

 drops, and a central region which is colourless and contains metachromatic 

 granules. The trichomes may be free-floating, or may form a more or less 

 mucilaginous mass on solid surfaces. Heterocysts occur only in certain 

 families. 



Reproduction is by means of hormogones, short lengths cut off from the 

 parent filament which are capable of growing into mature trichomes. Thick- 

 walled resting spores or akinetes are often developed, usually next to the 

 heterocysts. No method of sexual reproduction occurs. 



We shall consider one common type — Nostoc. 



Nostoc 



There are a number of common species which occur either terrestrially 

 or as aquatics in mucilaginous masses (" moon-spit "), living either attached 

 to a substratum or free floating. Each mass is composed of a more or less 

 firm jelly or mucilage, with a denser surface layer, inside which are numerous 

 twisted trichomes, which are yellowish-green in colour. These gelatinous 

 masses may be either solid or hollow and as much as several centimetres in 

 diameter ; they frequently break open to form flattened expanses with 

 lacerated margins. 



The trichomes are much contorted and form an intricately woven mass 

 which is more crowded towards the exterior (Fig. 89). Each is composed 

 of nearly spherical cells which are only loosely joined together. The 

 heterocysts are intercalary and solitary, and are oval in shape and slightly 

 larger than the vegetative cells (Figs. 90 and 91). 



Reproduction 



Hormogone formation is very common and results from the rupture of 

 the trichome at points where a vegetative cell and a heterocyst join. The large 

 numbers of trichomes in a colony is probably due to the development of the 

 hormogones into fresh trichomes without being liberated from the colony. 



