12 



PLANT MORPHOLOGY 



kind, called an akinete, arises from a v^egetative cell that enlarges by the 

 accumulation of food and develops a thick cell wall (Fig. 3A, B). These 

 cells are very resistant to unfavorable conditions. Akinetes may be 

 separated in the filament or several may occur together. In some forms 

 they always appear next to a heterocyst, either at the end or in the 

 middle of the filament. Another kind of resting cell, called an endospore, 

 is developed in some genera, as in Chamae siphon and in the marine genus, 

 Dermocarpa (Fig. 3C). Endospores are small thick- walled spores that 



B C 



Fig. 3. Formation of resting spores in the blue-green algae. A, portion of filament of 

 Anabaena with a heterocyst and an akinete containing many food granules, X750; B, 

 Gloeotrichia, showing a young filament and two stages in the development of an akinete, 

 X500; C, Dermocarpa, an epiphytic form, with two empty cells and others containing 

 endospores. (C, aftei- Bornet and Thuret.) 



arise from a protoplast by divisions within the cell cavity and from which 

 they later are liberated. 



Rivularia is a filamentous form in which the basal cell of a filament is 

 always a heterocyst, while the other cells become gradually smaller 

 toward the very slender apex. A thick mucilaginous sheath, confined 

 to the base of the filament, begins next to the heterocyst. Gloeotrichia 

 is similar to Rivularia except that the first basal vegetative cell becomes 

 transformed into an elongated akinete (Fig. 35). 



Branching. In some of the filamentous members branching occurs 

 (Fig. 4). In Tolypothrix the filaments exhibit "false branching." 

 Here the cells on one side of a heterocyst grow out beyond it to form 

 a branch. In Scytonema the false branches arise laterally in pairs but 



