SECT. II 



PHYSIOLOGY 



211 



we have here another example of symbiosis, in which the Leguminosae 

 furnish carbohydrates to the Bacteria, which, in turn, possess the power 

 of taking up free nitrogen, and passing it 

 on to the host in an available form (p. 1 7 3). 

 This is at least certain ; the Leguminosae 

 with such tubercles contain at maturity 

 more nitrogen than could have been pro- 

 cured from the nitrates and other substances 

 in the soil in which they grow. 



In addition to increasing the supply of 

 nitrogen, the presence of Rhixobium seems 

 to exert a favourable influence on the growth 

 of its host-plant. Peas and Lupines do not 

 thrive well in even the richest soil, if it 

 has been sterilised, and the formation of 

 the tubercles prevented. On the addition 

 of other unsterilised soil in which the Bac- 

 teria are known to exist, the tubercles will 

 then appear on the roots, the plants become 

 at once stronger, and show by their in- 

 creased growth a greater activity of their 

 metabolic processes. 



While parasitism or saprophytism is of 

 rare occurrence among the higher plants, 

 and confined to single species, in which it 

 often occurs only irregularly and is depen- 

 dent upon the environment, among the 

 lower plants it is more general; large 

 families with innumerable genera and species 

 are found completely devoid of chlorophyll 

 (Fungi and Bacteria), and altogether para- 

 sitic or saprophytic in their mode of life. 

 Of the Fungi and Bacteria some are true parasites, and are often 

 restricted to certain special plants or animals, or even to distinct 

 organs; others, again, are strictly saprophytic in their habit, while 

 others may be either parasitic or saprophytic, according to circum- 

 stances. What renders the conduct of these lower organisms parti- 

 cularly striking, is the peculiarity possessed by many of them of not 

 fully utilising all of the organic matter at their disposal ; but, on the 

 contrary, so decomposing and disorganising the greater part of it by 

 their fermentative activity that their own development soon becomes 

 restricted. When Yeast-fungi develop in a litre of grape-juice 

 they use very little of it for their own nourishment, but by far the 

 greater part of it becomes decomposed by the fermentation they 

 induce. As a result of this fermentation, together with the pro- 

 duction of carbonic acid, the grape-sugar solution becomes converted 



Fig. 1S6. — A root of View, Faba, 

 with numerous root -tubercles. 

 (Reduced.) 



