BY JOHX MCLUCKIE. 321 



tubercle-root is perhaps slightly larger (Text-fig. lb). The tubercles develop 

 almost without exception upon the uppermost secondary roots in nature, and 

 are thus close to the soil-level, or actually project above it, into the air (Text- 

 fig. 1«). The tubercles are negatively geotropic. 



In the seedlings of Macrozarma spiralis there is developed a strong, fleshy, 

 positively geotropic tap-root, into which the starch of the endosperm of the seed 

 is transferred. This root later develops a thin net-like covering of cork, on the 

 surface of which numerous bacteria and other micro-organisms may be found. 

 Secondary roots develop from the main tap-root — those towards the apical re- 

 gion of the root being perfectly normal — but the secondary roots formed near 

 or at the soil-level are frequently tubercle-like in form (Text-fig. 2). It is only 

 very occasionally that the more deeply seated secondary roots become tubercular. 



The root tubercles are not developed in seedlings which have been raised 

 on sterilised soil. A number of seeds were carefully washed for two minutes 

 in a sterilising solution of the following composition : mercuric chloride 1 gTam, 

 strong hydrochloric acid 3 c.c, water 200 c.c. Tiey were then rinsed for a time 

 in distilled water. Half of the seeds were placed in a pot of sandy soil (de- 

 rived from the natural habitat of Macrosamia spiralis) which had been thoroughly 

 sterilised by alternate beating and cooling. The soil was then watered with dis- 

 tilled water, and the pots placed in the laboratory. 



The remainder of the seeds were placed in pots of sandy soil which was 

 unsterilised. The soil was sprinkled with distilled water and pots placed with 

 the others in the laboratory. 



None of the seedlings raised in the sterilised sand developed the tubercles, 

 while approximately 70 per cent, of the seedlings in the unsterilised sand formed 

 tubercles. 



The result of this experiment and the fact that under natural conditions only 

 a proportion of the seedlings form tubercles seem to furnish quite conclusive 

 evidence that the peculiar tubercles are developed only as the result of bacterial 

 infection. This hypothesis, however, appears to be demonstrated by the follow- 

 ing experiments. 



Seedlings growing in sterilised soil were inoculated with Macrozamia bac- 

 teria from a pure serum-agar culture, and watered with distilled water. 



Another series of seeds were thoroughly sterilised, and germinated in a 

 large, sterilised, glass-stoppered jar; the main tap-root was inoculated and the 

 seedlings replaced in the jar which contained a piece of moist cotton wool to 

 maintain a fair humidity. In both experiments, all the seedling-s developed 

 tubercles of the usual form in from three to four weeks. 



The normal secondary roots of older seedling's were also inoculated, and 

 the seedlings kept in a moist atmosphere in a jar. In about a month from the 

 time of inoculation tubercles developed upon them: (Text-fig. 3). 



At first the tubercles were plagiogeotropic, but later became negatively 

 geotropic. The roots frequently remain unbranched, but others branch dicho- 

 tomously. 



In a transverse section (Text-fig. 4) of the root-tubercles there is a central 

 diareh stele composed of alternating xylem and phloem groups, surrounded by 

 pericycle and an endodermis whose radial walls are clearly defined by Karspare 

 strips. Surrounding the stele there is a very extensive cortex, of more or less 

 rounded, thin-walled cells with fairly considerable intercellular spaces and con- 

 taining protoplasm, a large nucleus, many small rounded starch grains, and in 



