Dependent Plants lOI 



by living plants again. So the large trees and beautiful 

 flowering plants are quite as dependent upon saprophytes as 

 the parasites and saprophytes are upon green plants. 



On roots of legumes or the pea family swellings or tubercles 

 are formed. They are swollen lateral roots containing 

 minute bacteria which make their way from the soil through 

 the root hairs into the roots. They are plants which obtain 

 nourishment from their host, but they enable the host to use the 

 free nitrogen which is abundant in the air. Nitrogen is a valu- 

 able and expensive food material, and can be obtained by 

 most plants only from compounds in the soil. Legumes can 

 not only obtain it for themselves, by the help of these little 

 bodies, but they leave nitrogen compounds in the soil, to be 

 used by other plants. Just think of all the plants in this 

 country which have pods and belong to the pea family ! How 

 they are enriching the soil ! ^ Yellowwoods (Podocarpus) have 

 little plants living in their roots which perform the same 

 service. 



A lichen (see page 4), is made up of two plants, a green 

 alga which manufactures the food, and a colourless plant, a 

 fungus which provides the alga with the necessary salts and 

 water. The threads of the fungus grow around and cunningly 

 enclose the alga, which is thus prevented from drying up in the 

 exposed situations which lichens frequent. 



Plants which live together in this way and are helpful to 

 each other are called Symbionts. Plants are sometimes 

 symbiotic with animals. There is a kind of Acacia with little 

 holes in the base of the large hollow thorns. Within these 

 thorns ants make their nests. Other insects eat and injure the 

 leaves of the Acacia. The leaves manufacture a nectar which 



^ It is for this reason that farmers in winter sow peas in their vineyards 

 and orchards and plough them under in spring. They should be ploughed 

 under before the seeds have ripened. That leguminous plants enriched 

 the soil was known in the time of Pliny, but the myslery was not solved 

 until 1886, by the scientists Hellriegel and Wilfarth. The group of 

 plants thus indebted to bacteria is larger than was at first supposed. 

 Bacteria for inoculating soils or seeds for different crops can now be 

 bought and the yield of grains and tomatoes as well as of legumes has 

 been much increased by bacterial inoculation. 



