IRREGULAR NITRITION 20; 



has been seen that in pure culture the Bacterium can in some dcKrc-r 

 fix free nitrogen from the air. The roots would supply the sugar and 

 proteids necessary for the process. Possibly the coalition of the two 

 hving organisms in some way encourages it. Finally the digestion 

 of most of the " Bacterioids " by the living cell in which they had 

 developed, secures the transfer of the nitrogenous compounds acquired 

 to the higher plant. This is again a case of " Phagccytosis,'* but here 

 it follows on active fixation of nitrogen from the atmosphere. The 

 importance of this in Agriculture is obviously very great, for ths supply 

 of combined nitrogen is the most difficult problem of the farmer. 



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 VIII. 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 humus soil, where the native Pingniciila and Drosera are 

 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, 

 glandular cells. This has been studied very thoroughly in the Sundew 

 {Drosera), m which the mechanism of capture has already been 

 described (p. 131). The secretion which plays so important a part 

 is produced by the glandular head of the tentacle, which receives its 

 vascular supply through a strand traversing its stalk. (Fig. 1 57.) The 

 viscid secretion is exuded by the epithelium that covers the surface 

 of the gland. On contact with an insect, or with a small piece of 

 nitrogenous matter, such as a cube of white-of-egg, the gland is 

 stimulated to greater secretion. A proteolytic ferment, which breaks 

 down the complex proteid to simpler soluble substances, is given out, 

 and the secretion takes an acid reaction. The emission of the ferment 

 and of the acid is, as in the gastric secretion of the animal stomach, 



