STUDIES IN SYMBIOSIS. 



II. The Apogeotkopic Roots of Macrozamia spiralis and their 

 Physiological Significance. 



By John McLuckie^ M.A., D.Sc, Lecturer in Plant Physiology, University 

 of Sydney. 



[Read 26th July. 1922.] 



Introduction. 



Since Reinke (1872) published his account of the coral-like roots 

 of Cyca^ revoluta, considerable interest has been taken in these peculiar 

 structures. Reinke described an endophytic Anabaena associated with the root- 

 tissues of Cycas revoluta. Schneider (1890) also studied these peculiar roots, 

 which he called "tubercles." He described the association of the Algae and 

 Bacteria with the root-tissues as a mutualistic symbiosis. Life (1901) made a 

 further study of the tubercle-like roots of Cycas revoluta, and came to the con- 

 elusion that the dichotomy of the roots is apparent, not real, as a part of the 

 original meristem remains after the meristems of the two branches have been 

 definitely established. This residual meristem, however, does not function fur- 

 ther, and soon disappears, so that the older bifurcated roots show what appears 

 to be a true dichotomy. 



He isolated on agar cultures, three bacteria from these tubercles. Hyphae 

 of a fungus not identified were observed. A Nostoc-like Alga was also observed 

 in a definite "Algal zone." The tubercular-roots of Cycas revoluta do not possess 

 a true root-cap but, instead, "a sheath of several cell layers extends over the 

 tip, and envelops the entire tubercle as an outer cortex." Numerous lenticels 

 were observed on the tubercles, and Life concluded that the tubercles of Cycads 

 have at least two functions, namely aeration and assisting in nitrogen fixation. 



Zach (1910), in a later study of the roots, concluded that the fvmg-us pre- 

 sent is not a syjnbiont, but a parasite against which the cell reacts as a phagocyte. 



Bottomley (1909) isolated and gTCw the nitrogen-fixing organisms, Pseu- 

 domonas radieicola and Agotobacter from Cycas tubercles. It therefore appears 

 certain that, in Cycas at any rate, the root tubercles, by virtue of the bacterial 

 organisms present in them, are capable of nitrogen fixation. 



Observations on Macrozamia spiralis. 



The investigations outlined above deal only with the tubercles of Cycas, 

 but although tubercles* of a somewhat similar form occur on Macrozamia 

 spiralis, I am not aware of this fact having been previously recorded. The 



*I have used the term "tubercle" to signify the apogeotropic roots. 



