Symbiosis 35 



a temperature of possibly 10 less. Of the Chroococcace.-P, Aphanothece 

 thermalis Brugger and A. bullosa, Rabenh. have been observed living in 

 water at a temperature of 6<S75 C. On the whole, it may be said that 

 no organisms, with the possible exception of certain Bacteria, possess such 

 a capacity as the Myxophycere for withstanding extremes of temperature. 



Myxophycetp of the genera Gloeocapsa and Gloeothece are responsible 

 for the formation of oolitic calcareous grains on the shores of the Great 

 Salt Lake, Utah, and white grains of a similar kind are known from the 

 Red Sea. On the bottom of Lough Belvedere, near Mullingar in Ireland, 

 numerous spherical calcareous pebbles have been found, which have been 

 gradually built up by the growth of Schizothrix fasciculata (Murray, '95) 

 Similar pebbles are also known from other places, both in Europe and 

 N. America. It is not improbable that the oolitic particles of rocks of 

 various ages may have had a similar origin, and that the structure described 

 as Girvanella problematica consists of the mineralised sheaths of an ancient 

 genus of the Myxophyceae. It is also possible that Zonatrichites, described 

 by Bornemann from a breccia of the Keuper age, in Silesia, is a fossil 

 member of the Rivulariacese which caused the construction of more or less 

 hemispherical calcareous nodules (Seward, '98). When due consideration 

 is given to the precisely similar nodules which are formed by Diclwthrix 

 Ul/psophila (vide fig. 21), and other living members of the Rivulariacea^, the 

 view that Zonatrichites was one of the Myxophycea? is to some extent 

 supported, notwithstanding the complete absence of cellular structure. 



The living genus Hyella is a perforating Alga on the surface of the 

 calcareous shells of Molluscs. 



SYMBIOSIS. The curious endophytic Anab&na Azollx Strasburger lives 

 and grows in certain slime-filled hollows in the leaves of Azolla, into which 

 cavities project hairs from the living layer of cells. It has been suggested 

 that this is a case of symbiosis. The Anab&na is always present in the 

 cavities and the terminal parts of the hairs become colourless and finally 

 more or less disintegrated. 



Anabwna Cycadearum Reinke is another endophytic species which lives 

 within the modified tubercle-like roots of species of Cycas. The Alga 

 occupies a well-marked intercellular-space-zone just within the cortex 

 (fig. 22), and is associated with the two nitrogen-fixing Bacteria, Pseudo- 

 monas radicicola and Azotobacter. 



Horejsi ('10) has given a good account of the distribution of the Alga 

 in the cortex of the root, and he showed that by its penetration into the 

 apical meristem it stopped normal growth and caused dichotomy. Thus 

 the peculiar tubercle-like roots of Cycas and other Cycads are actually caused 

 by the endophytic Alga. He found that the Alga maintained its existence 



32 



