328 STUDIES IN SYMBIOSIS, ii. 



The growth of the root tubercles is clue to infection by soil bacteria, of 

 which two forms are generally present in the sheath and cortical cells. 



The presence of the bacteria in the cells of the root stimulates the develop- 

 ment of the cortex and sheath, so that the tubercles are always more massive 

 than ordinary roots. 



Zoogloea-threads of bacteria arise in many cells of the cortex, and the bac- 

 teria are frequently crowded in the vicinity of the nucleus. The starch-content 

 decreases in the cells containing bacteria, and is probably used by them in the 

 process of nitrogen fikation. 



Occasionally unicellular algae may be present amongst the outer dis- 

 organised cells of the sheath, but there is no definite "algal-zone" as in Cijcas 

 revoluta. 



The bacteria are not nitrate-formei-s but assimilate free nitrogen and render 

 it available in a combined form for the plant. 



The structure of the tubercles also indicates that they may function sub- 

 sidiarily in aeration. 



Similar root-tubercles occur in other species of Macrozamia, e.g.. M. coral- 

 lipes. 



I desire to record my sincere thanks to Professor Lawson for his helpful 

 criticism. 



Literature. 



Reinke, J., 1872. — Parasitische Anabaena in Wurzelen der Cycadeen. Gottingen, 

 Nachrichten, p. 107. 



Schneider, A., 1894. — Mutualistic symbiosis of Algae and Bacteria with Cycas 

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Life, A. C, 1901. — The tubercle-like roots of Cycaa revoluta. Bot. Gaz., 31, 

 265—271. 



Zach, F., 1910. — Studie uber Phagocytose in den Wurzelknollchen der Cyca- 

 deen. Oest. Bot. Zeitschr., 60, 49-55. 



BOTTOMLEY, W. B., 1907. — The Structure of Root Tubercles in Leguminous and 

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NOBBE and Hiltxer, 1899. — Die endotrophe Mykorrhiza von Podocarpus und 

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Spratt, E. R., 1912. — The Formation and Physiological Signiflcanee of the Root 

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Vines, 1888. — On the relation of the formation of tubercles on the roots of 

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 386—389. 



Frank, 1885. — Ueber die auf Wurzelsymbiose beruhende Enahrung guersser 

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