BY JOHN MCLUCKIE. 323 



The intercellular space-system of these tubercles is extensive and com- 

 municates with the atmosphere through the loosely arranged sheath-cells. Lenti- 

 cels, which Life (1901) has described in Cycas revoluta, are not developed on 

 the tubercles of Macrozanuia spiralis. "Nevertheless, gaseous exchange between 

 the atmosphere and the cortical tissues of the tubercles is provided for by the 

 very effective intercellular space system of sheath and cortex. In sections of 

 fresh material mounted in water, a considerable amount of air is present in the 

 spaces. 



These tubercles differ from normal secondary roots in certain respects. In 

 the former the tip is rounded, not conical, while the "root-cap" appears as a 

 persistent, continuous sheath of radially elongated, and somewhat papillate cells. 

 Life observed a similar sheath in Cycas revoluta and regarded it as an outer 

 cortex. Morphologically the sheath in Macrozamia appears to be analogous to 

 the velamen of the aerial roots of some orchids, and notwithstanding the fact 

 that the calyptrogen and dermatogen cannot be clearly defined as distinctive 

 cell-layers, the e\'idence supports the view that this sheath is the root-cap which, 

 however, has been considei-ably transformed and disturbed, like the meristem 

 itself by incursions of bacterial organisms. 



The bacteria which occur in the tissues of these tubercle-roots are no doubt 

 soil-forms which enter through some rupture in the main root where a secondary 



Text-fig. 6. — A longitudinal section of a dichotomously branched 

 tubercle showing sheath (s), cortex (c), plerome (p), and 

 a mass of meristematic cells (m) . (x 40). 



Text-fig. 7.— The bacteria of Macrozamia tubercles isolated and grown 

 upon a serum-agar medium, (x 1000). 



root is growing out, or which gain access to the primary root from the groc 

 the hypocotyl. Unless infection takes place, the secondary roots show n 

 development. 



