IRREGULAR NUTRITION 



239 



It has been established that the red colour which the central tissue of the 

 nodule usually shows is due to haemoglobin. 



Root nodules are not entirely restricted to the family Leguminosae, but are 

 also found for example on the Alder tree, Bog Myrtle, Sea Buckthorn, and a 

 few other plants. The infecting organisms here are different from that in 

 leguminous types, but they also possess the faculty of nitrogen fixation. 



Carnivorous Plants. 

 The predatory methods of Carnivorous Plants have been considered 

 from the point of view of the reception of stimulus, and the consequent 

 movements, in Chapter IX. Another aspect of them is in respect 

 of nutrition. The plants which show this peculiar habit grow under 

 conditions where the supply of combined nitrogen is difficult, such as 

 in peaty or humous soil, where the native Pinguicula and Drosera are 



B 



Fig. 166. 



Leaves of Drosera rotundifolia : enlarged. A , in the receptive state before 

 stimulation. B, after stimulation, viewed from above, with tentacles partly 

 incurved. (After Darwin.) 



habitually found. Venus' Fly Trap and Sarracenia grow on boggy 

 moors in America, while Nepenthes is an epiphyte. In such positions 

 nitrates and other salts would naturally be deficient. On the other 

 hand, all of these plants contain chlorophyll, and can thus construct 

 carbohydrate for themselves. In point of fact the carnivorous habit 

 is not essential to their existence, though experimentally it is found 

 that a moderate supply of animal food is beneficial. 



Apart from the mere mechanism of capture, which is very various, 

 the physiological treatment of the prize is fairly uniform. Its success 

 depends upon secretion of digestive juices from certain localised, 



