326 ECOLOGY ' 



in being able to assimilate carbon dioxide in darkness. To 

 carry on their work, they not only need suitable mineral 

 food, but also free oxygen and moisture. Their action is 

 stopped by too dry a soil and by strong sunlight. From 

 this we sec why aeration of the soil is essential, and why 

 over-watering, which drives out the air, is injurious. 



An exception to the rule as to the source of nitrogen for 

 green plants is found in members of the Pea family (Legu- 

 minosae) and a few others, such as the Alder and Sea Buck- 

 thorn. On the roots of these plants, tubercles are formed 

 (Fig. 256, 3 in) as a result of attacks by bacteria-like organisms 

 in the soil, which enter by the root-hairs and cause the 

 swellings. These organisms fill the tissues of the nodules 

 and are able to take up the free nitrogen of the air and con- 

 vert this into nitrogenous compounds, which in turn are 

 passed on to the ' host '-plant as food, or given up to the 

 soil when the plants decay. By virtue of this alliance, the 

 nodule-bearing plants are able to thrive in soils deficient in 

 nitrates. Such a union of organisms is called symbiosis (see 

 p. 356). If soil, deprived of nitrates by previous crops, is 

 sown with Clover or other leguminous plants, and the latter 

 ploughed in as green manure, the loss is made good and the 

 land becomes richer. Another exception is found in insec- 

 tivorous plants, which are able to supplement their supply 

 of nitrogen from animals which they capture and digest 

 in peculiarly modified leaves (see p. 361). 



Test for Nitrates. Dissolve a little potassium nitrate in 

 water, add three or four drops of diphenylamine sulphate, 

 and then a little strong sulphuric acid. The deep blue 

 colour produced indicates the presence of nitrates. Test 

 a sample of garden soil for nitrates. First dry the soil, 

 plug the tube of a funnel with cotton wool, filter paper, or 

 asbestos, and fill up with the dry soil. Carefully pour 

 water on it, and collect a small quantity of the water which 

 drops from the end of the funnel, and test for nitrates as 

 before. 



