io6 A TEXTBOOK OF THEORETICAL BOTANY 



Reproduction may take place by the breaking of the trichome, within the 

 sheath, into short filaments of cells which are termed hormogones. The 

 points of disjunction are marked by collapsed biconcave cells. The hormo- 

 gones are set free by a breakdown of the sheath at the end of the parent 

 filament. They are capable of slow movement though they have no flagella, 

 and finally settle down and form fresh filaments. In one or two species 

 reproduction may be effected by gonidia, which are tiny non-motile cells 

 derived by the division of one or more cells of the parent plant. These are 

 liberated and finally grow into fresh filaments. Resting spores may also 

 be produced. These bodies arise from vegetative cells which increase in 

 size, become gorged with reserve material, and are often yellow or 

 brown in colour. They are frequently developed in association with 

 heterocysts. The unicellular species generally multiply only by simple cell 

 division. 



Many species of the Cyanophyceae are world-w4de in distribution. They 

 are among the first organisms to colonize new ground and they were the 



Nostoc colony 



Fig. 88. — Gimnera Diankata. Transverse section of the 

 underground stem showing colonies of Nostoc em- 

 bedded in the tissues. 



first organisms found on the island of Krakatau after the eruption of 1883. 

 They are found in the Arctic, even though they may be covered for months 

 on end with snow and ice. On the other hand, as we have seen, they occur 

 very commonly in hot springs. Some species live within the tissues or even 

 within the cells of higher plants. Species of Anabaena are found in cavities 

 m the fronds of the water-fern Asolla, and also in the roots of the gymno- 

 sperm Cycas. Species of Nostoc form colonies in the underground stem of 

 the giant angiospermic marsh plant Gumiera manicata (Fig. 88). Many 



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