544 



THE PATHOGENIC ALGAE. 



forme (the earlier name) ; he also believes that it exists on 

 damp soil and independently of (imnu'ra. 



Anabaena cycadearum (Reinke). 1 [Nostoc commune 

 (Schneider). 2 ] The following account of this species is taken 

 from De Bary. 3 Seedlings of Cycadeae have a thick tap-root 

 which branches in the soil : from the proximal end of the 

 primary root a few pairs of root-branches grow up perpen- 

 dicularly, and, after forking once or twice, their ends swell to 

 form tubercles (Fig. 321). Similar clumps of forked twigs 



FIG. 322. Anabaena cycadearum. Section of a Cycad-root, as in Fig. 321 A, 

 and in the condition shown in the upper transverse section B of that figure. 

 The ^oxtoc-filanients crowd the intercellular spaces and cause alteration in shape 

 and growth of the adjacent cells as shown, (v. Tubeuf del.) 



arise later on other branches which arise from the tap-root 

 and spread over the surface of the ground. 



It is into these forked twigs that the Nostoc makes its way 

 and causes the following characteristic alteration in their 

 structure. A layer of parenchyma, which in normal roots does 

 not differ from the surrounding compact polygonal tissue, becomes 

 in attacked roots a definite zone round the axile vascular bundle. 

 The zone consists of parenchymatous cells much elongated in one 

 direction, and with their interspaces filled with masses of algal 

 filaments (Fig. 322). In cross-sections of attacked roots the Nostoc 



Weinke, Botan. Zeitmuj, 1879, and Abhandlwigen, 1873. 



2 Schneider, Botanical Ga-ctte, 1894, p. 25. 



3 De Bary, Die Erscheinung der Symbiose, 1879, p. 14. 



