264 THE BIOLOGY OF FLOWERING PLANTS 



are very complex. In most genera a secondary alteration 

 in structure produces a large internal space in the tubercle in 

 which lives the blue-green alga Anabaena, The significance 

 of its presence is not known. In addition there is present 

 Bacillus radicicola and also Azotobacter chroococcum, a nitro- 

 gen-fixing bacterium found otherwise only in the soil. We 

 have here the remarkable case of a fourfold symbiosis of 

 Cycad, Anabaena, Azotobacter, and Bacillus. 



Leaf Nodules. — A still more intimate bacterial symbiosis 

 is found in the leaves of certain tropical Myrsinaceae, 



Fig. 35. — Infection of root hairs of Phyllocladus by Bacillus radicicola. 



X 480. (After Spratt.) 



e.g. Ardisia crispa, and some other species of this genus, 

 Ambylanthus and Ambylanthopsis, and among Rubiaceae, 

 e.g. Pavetta, Psychotria, Grumilea, Ixora. The symbiosis 

 in the Rubiaceae was first discovered by Zimmermann 

 (1902), and later investigations are due to Faber (1912, 1914). 

 The investigation of Ardisia has been carried out by Miehe 

 (1911, 1914, 1919). (Cp. also Orr, 1923, on Dioscorea.) 



In Pavetta the bacterium, Mycobacterium Rubiacearum, 

 causes the formation of small knots or galls on the leaves. 

 It also lives free in the stipular cavity of the embryonic 

 leaves, at the vegetative point. As each leaf unfolds, 



