134 



THE EVOLUTION THEORY 



captives, for the valve does not open outwards ; therefore they soon 

 die and decompose, and are then taken up by special absorptive cells 

 (B, C, sz) and utilized as nourishment for the plants. In this way 

 the Utricularia3 catch numerous little crustaceans and insect larvae, 

 which slip into their traps, presumably for concealment. 



Another example is found in the marsh plants of the genus 



Nepenthes, some species of which 

 live as climbers on the outskirts 

 of tropical forests, climbing up the 

 trees and letting their long, thin 

 tendrils hang downwards, often 

 over ponds and stagnant pools, 

 where swarms of small flying 

 insects abound. These plants 

 have developed exceedingly re- 

 markable contrivances for catch- 

 ing insects and using them as 

 food (Fig. 24). The long stalks 

 (St) of their leaves (Spr) are 

 first bent downwards, then they 

 suddenly turn sharply upwards, 

 and the upturned portion is modi- 

 fied into a pitcher-like structure, 

 in the bottom of which a fluid 

 gathers, acid in taste, containing 

 pepsin, and therefore a digestive 

 fluid. Nitrogenous substances, 

 such as flesh, dissolve in this fluid, 

 and insects which fall into the 

 pitcher from the rim are killed 

 and dissolved. There are many 

 species of Nepenthes, but not all 

 of them possess the trap -structure 

 in equal perfection, so that Ave 

 are able, to some extent, to follow 

 the course of its evolution, from 

 a broad leaf-stalk, somewhat bent 

 over at the edges, to the marvellous closed pitcher shown by Nepenthes 

 villosa (Fig. 24) of Borneo. In this species the pitchers attain a length 

 of fifty centimetres, and are beautifully coloured, resembling in that 

 respect, as well as in their form, the tobacco- pipe-like flowers of the 

 tropical Aristolochire. When we come to discuss the origin of flowers, 



Fig. 24. Pitcher of Nepenthes villosa, after 

 Kerner. St, stalk of the leaf. Spr, its apex. 

 Fk, the pitcher. R, the margin beset with 

 incurved spines. 



