THE CYANOPHYCEAE. 541 



relationship. The manner in which typical chlorophyllous plants- 

 gradually become shelter-parasites, and pass from this into the 

 condition of true parasites, is well demonstrated amongst the , 

 algae. Few of the parasitic algae can be said to affect their 

 host injuriously by causing death of its tissues ; Phyllosiphon is 

 the most marked case of this kind. 



A. THE CYANOPHYCEAE. 



These, the blue-green algae, possess a homogenous bluish 

 green plasma, with a colouring matter consisting of phycocyan 

 and chlorophyll. Multiplication takes place only by cell- 

 division ; sexual reproduction does not occur. Many of the 

 forms are adapted to a symbiotic life, yet without prejudice to 

 their ability to live as independent organisms. Numerous 

 species form lichens. They are in many cases capable of long 

 resistance to drought. 



The Cyanophyceae are common both as endophytes and 

 epiphytes on other algae and on higher plants. Amongst them 

 will be found examples of purely shelter-parasitism, of true 

 parasitism, and all intermediate stages, yet no cases are known 

 of real injury or death to host-plants resulting from members 

 of this group. Amongst forms endophytic in Phanerogams may 

 be noticed Nostoc gunnerae in Angiosperms, and Anahaena 

 cycadearum in Gymnosperms ; in Pteridophyta only Anahaena 

 azollae is known, and in Bryophyta, Nostoc lichenoides. In every 

 case the Nostoc penetrates as a shelter-parasite into fissures or 

 cavities already existing in the host, and becomes as a rule 

 entangled in a slime occupying the cavity. The Nostoc in 

 Gunnera becomes parasitic at a later stage, and makes its way 

 into the cavity of cells. The other species are never endophytic 

 in the host-cells, though they may affect the cells surrounding 

 a cavity and stimulate them to further growth, apparently, 

 however, without any injurious effect on the host-plant. 



Nostoc punctiforme (Kutz.) P. Harlot^ {Nostoc {Scytonema) 

 gunnerae Eeinke).^ This occurs in various species of Gunnera, 

 natives of South Africa, New Zealand, and South America ; or 

 cultivated frequently in Europe. The occurrence of the Nostoc 

 is in every case the same, its presence being indicated to the 



'Hariot, Compi, rend, cxv., 1892. 



^Reinke, Morphologische Ahliamdlungen, Leipsic, 1873. 



