282 THE BIOLOGY OF FLOWERING PLANTS 



certainty. It has arisen, as we have seen, at least twice in 

 the course of evolution. The variety of mechanism dis- 

 played suggests a still greater diversity of origin. 



There are two main types of organisation. The pitcher 

 is exhibited by the various pitcher plants, and of a some- 

 what similar nature is the bladder of Utricularia. Now 

 pitcher formation on leaves is not uncommon as a sport 

 in many plants, e.g. the cabbage. It is Hkely that the 

 permanent acquisition of such bladders or pitchers was 

 the first step, and that greater specialisation followed. The 

 second type is the viscid gland type of the Droseraceae and 

 Pinguicula. Hairs with a sticky secretion are common in 

 many families, e.g. the Saxifragaceai with which the 

 Droseraceae are probably allied. Insects are freely caught 

 by such hairs, and this may have been the foundation of 

 the insectivorous habit along this line of organisation. 



It is interesting to note that both types are represented 

 in each of the two widely separated groups of insectivores. 



In these modes of abnormal nutrition (except in com- 

 pletely parasitic and saprophytic forms which are wholly 

 dependent on other organisms), the exact relations are in 

 many cases obscure, but in very many the supplementing 

 of the supply of nitrogen seems to be the chief advantage 

 secured by the higher plant. This is so in many mycorhizal 

 plants, in the plants with bacterial symbiosis, and in the 

 insectivores. These are all predominantly inhabitants of 

 soils naturally poor in nitrogen compounds ; the vegetation 

 of peaty soils includes examples of all three groups. Nitrogen 

 is the element the supply of which is most precarious, and 

 this varied display of special means to secure it emphasises 

 the difficulty and the importance of obtaining a sufficient 

 supply. 



