624 Spratt.—The Rool-Nodttles of the Cycadaceae. 
(c) the separation of two algal zones by a meristem which has been observed 
in Stangeria. 
The Anabaena would naturally confine itself to an area not many cells 
deep, if photosynthesis were to go on, because of its dependence on light, 
the intensity of which must be rapidly diminished as it passes through the 
overlying cells. It is also impossible for the algal zone to be developed in 
nodules situated far below the surface of the soil, to which the photo¬ 
synthetic rays of the sun cannot penetrate. In those cases in which the 
algal zone appears to be entirely absent it may be due to the method 
of cultivation, possibly to the absence of the specific Anabaena from the 
soil, since nodules are produced so abundantly without it and correspond 
very closely with those of other genera in which it afterwards develops. 
The algal zone is an area in which photosynthesis is taking place and 
probably attracts chemotactically the nitrogen-fixing organisms which 
require a carbohydrate as their source of energy. The three organisms 
thus collect together and work symbiotically, undoubtedly benefiting the 
cycad by the rich supply of elaborated material they produce. 
In conclusion my thanks are due to Professor W. B. Bottomley for his 
sympathy and suggestions during the progress of these investigations. 
Summary. 
1. All the cycadean genera produce root-nodules which are perennial, 
modified lateral roots, repeatedly branched and typically forming large 
coralloid masses. 
2. They are produced primarily by infection with Bacillus radicicola . 
3. A whorl of lenticels or a continuous zone of loosely arranged paren¬ 
chymatous cells is produced at the base of each nodule. 
4. The outer cells always become pushed apart and infected by 
Azotobacter and, if suitable conditions prevail, by Anabaena also. 
5. The presence of the Alga stimulates the phellogen to produce 
other lenticels, from which and the basal area a zone of tissue is produced 
which encloses the original outer cells in which are the Alga and Bacteria. 
6 . The algal zone is continuous, except immediately below the lenticels, 
extending from the base nearly to the meristematic apex. 
7. The algal zone consists of a large air-space containing Anabaena 
and Azotobacter which is kept intact by papillate cells traversing it from 
both the inner and outer tissues. 
8. Bacilltts radicicola is chemotactically attracted to the algal zone; 
thus leaving the cortical cells in which large quantities of starch grains 
and sphaeraphides are deposited, and in Dioon> also tannin. 
9. No algal zone has been observed in Macrozamia , Zamia , Cerato - 
zamia> and Bowenia , but nodules are produced containing Bacillus 
radicicola and Azotobacter. 
