196 



FLOWERLESS PLANTS 



(d) Plants are, as has been just remarked, not able to take 

 their supply of nitrogen from the air which has such an abundant 

 quantity of it (about 79^/o); certain Bacteria form, however, an 



exception. They grow as 

 parasites on the roots of the 

 Leguminos^e and form no- 

 dules on them (cf. Pea, page 

 35). These nodules, when 

 squeezed, throw out a sticky 

 fluid which really consists of 

 innumerable Bacteria that 

 can be readily recognized un- 

 der a powerful microscope. 

 TJie Bacteria that form these 

 nodules, are able to derive their 

 nitrogenous food from the air, 

 which higher vegetation is 

 unable to do. The larger 

 the quantity of root-nodules, 

 the greater the amount of 

 nourishment derived from the 

 air and stored in the soil. 

 The advantage of growing 

 Pulses, Sunn-Hemp or other 

 Leguminosie, to recoup the 

 land, is explained by the 

 peculiarity of these plants. 

 2. Certain other Bacteria 

 produce, in the substances on which they live, a change which 

 is not called decay, but fermentation {cf. page 22). Again, if 

 toddy or wine is allowed to stand open for a few days, it 

 becomes sour. This is also due to the action of some Bacteria. 

 Similarly it is the Bacteria which turn milk sour or spoil boiled 

 rice and vegetables. By the action of liacteria the fibres of 

 Sunn-Hemp (Crotalaria) are loosened; and also the peculiar 

 flavour of Cocoa and Tobacco is due to tlio influence of these 

 little organisms wliich cause fermentation. 



Fi«?. 17 9. — Root- 

 noclules of the Pea 

 plant (natural size). 

 C. Cell of tubercle 

 filled with innumer- 

 able Bacteria (120 

 times enlarged). 

 b. Bacteria (800 times enlarged). 



