AGRICULTURAL BOTANY, CHEMISTRY AND PHYSIOLOGY OF PLANTS IO9I 



849 - The Root Nodules of Ceanothus americana and of Cycadaceae. — i- 



BoTTOMLEY W. B., Tlic Root Nodules of Ceanothus americanus, in Annals of Botany, 

 VoL XXIX, N" 116, pp. 605-610 + pL XXVIII. London, 1915. — II. Spr.^tt H. R., The 

 Root Nodules of Cycadaceae. Ibid.; pp. 619-625 -{- pL XXIX. 



I. The writer examined root-nodules from species of Ceanothus growing 

 wild in North America, where the genus is indigenous. Nodule-bearing roots 

 of two different species were examined : Ceanothus americanus L- (New Jer- 

 sey tea) and Ceanothus velutmits Dougl. (mountain balm). As the prelimi- 

 nary examination showed that the nodules of both species are practically 

 identical, the following description of C. americanus applies equally to 

 C. velutinus. 



The root-nodules of Ceanothus americanus are modified lateral roots. 

 They are perennial and increase in size each year by the formation of endo- 

 genous outgrowths (branches) similar in structure to the primary branch. 



Each primary nodule and branch when fully grown shows four zones : 

 a) an apical meristematic zone ; b) an infection zone, where the cortical 

 cells are becoming infected with bacteria ; c) a bacterial zone, containing 

 many radially-elongated enlarged cells filled with Bacteria ; rf) a basal zone 

 almost free from bacterial cells. 



^y\ The younger bacterial cells contain rod-shaped organisms, the older 

 ones spherical bodies. These latter are the " bacteroid " condition of 

 the active nitrogen-fixing rod-shaped bacillus. 



The bacteria, when isolated atid gro\\Ti in pure culture, can fix free 

 atmo-spheric nitrogen, and from their structure, mode of growth and forma- 

 tion of " bacteroids " evidently belong to the Bacillus radicicola group. 



The paper is accompanied by a bibliography containing 7 references. 



II. — Amongst non-leguminous plants it is now recognized that the 

 Elaeagnaceae, Myricaceae, Podocarpinae, and the genus Alnus have root- 

 nodules, which are definitel}'' concerned with nitrogen assimilation. With 

 these the Cycadaceae must be associated, because BoTTOMLEYhas isolated 

 from the nodules of Cycas not only Bacillus radicicola but also Azotohacter, 

 both of which organisms are known to assimilate atmospheric nitrogen. They 

 are, however, of special interest because in their cortex a very definite 

 green ring, the algal zone, is produced by the presence of an Anahaena, 

 which has been described by the writer. 



The first part of the present series of investigations was carried out 

 using as material Cycas circinalis ly. and Encephalartos Hildehrantii A. Br. 

 & Bouche and was extended to comprise the genera S^aw-o-^Wa, Macrozamia, 

 Zamia, Ceratozamia, Dioon, and Boiu'enia. 



Root-nodules have been found to occur throughout all the genera of 

 Cycads, and as in other non-leguminous plants, thej- are perennial modified 

 lateral roots which have diverged from their normal growth owing to 

 infection with the nitrogen-fixing organism Bacillus radicicola. A whorl of 

 lenticels or a continuous zone of loosely arranged parench3'matous cells is 

 produced at the base of each nodule. The outer cells always become pushed 

 apart and infected by Azotohacter and, if suitable conditions prevail, by Ana- 

 hoena also. The presence of the alga stimulates the phellogen to ])ro(luce 



