6 THALLOPHYTA 



surface; the rods at first divide every half hour and subsequently more slowly, 



losing their motility after a day or two (I'ig- - l>) and forming a pellicle or 

 scum on the surface of the liquid (Fig. 2 «•). After a time spores arc formed 

 in the rods of the pellicle, these rods being usually united, at this Stage, 

 end to end in chains (Fig. 2 b and r). One spore is formed within each rod 

 and is subsequently set free by the disappearance of the cell-wall. 



It has of late years become a matter of common knowledge that the great 

 value of leguminous plants in increasing the nitrogen-content of the soil is 

 due to the bacteria (rhizobium) contained in the small tubercles always found 

 on their roots, these bacteria being able to absorb and utilize the nitrogen 

 of the atmosphere and to pass it on in an available form to the plant. 



Pure cultures of these bacteria are now sold under the names of nitragin 

 and azotogen, although their practical utility as soil-fertilizers is not admitted 

 by all bacteriologists, since the results obtained with them have proved 

 somewhat contradictory. 



It is a less familiar fact that the conversion of nitrogenous substances in 

 the soil into a form available for plant food, i.e. into nitrates, is also carried 

 on by bacteria. They belong to the genera Nitromoruis and .izotobacter, and 

 are specially valuable in dealing with sewage on so-called " sewage farms." 



It would take us too far to go into the question of inoculation against 

 diseases of bacterial origin by means of "anti-toxins" and similar substances. 

 It will suffice to point out that it has been found that culture at a higher 

 temperature (amongst various other factors) decreases the virulence of many 

 pathogenic bacteria with each succeeding generation. The products of such 

 weakened bacteria may then be used to impart the disease in a milder form, 

 after recovery from which the patient is usually immune to the ordinary form 

 of the complaint. 



This method has been applied successfully in the case of bubonic plague 

 and some other bacterial diseases, and present-day research in bacteriology 

 is largely directed towards such problems. 



III. SCHIZOPHYCEAE* (CYANOPHYCEAE). 

 The Blue-Green Algae. 



This is a group of small but widely distributed algae in which the pure 

 green colour of the chlorophyll is masked by another colouring matter, 

 frequently blue, but often red or yellow. The plants, as seen with the 

 naked eye, may be blue-green, olive-grey, nearly black, various shades of 

 red, orange or yellow. 



They occur in wet or damp situations, favourite habitats being water, 



still or running, containing a high percentage of organic substances in solution, 



and the wet vertical faces of rocks in waterfalls, etc. 



* Details of some algae from several classes collected in South Africa arc given by 

 N. Willk in his paper : " Ueber einige von J. Menyhardt in Suedafrika gesammelte 

 Suesswasseralgcn," Oesterr. Hot. Zeitschr. 1903, No. 3. A large number of fresh-water 

 algae recently collected by Prof. H. H. VV. Pearson in Namaqualand and Angola are 

 described by G. S. West in Annals S. A. Museum, vol. IX, pp. 61 — 89 (1912). 



