FLOWERING PARASITES 187 



to germinate unless they happen to lie in 

 close proximity to the host of a plant which 

 they can successfully attack. This striking 

 peculiarity enables us to appreciate some- 

 thing of the remarkable qualities which 

 render so specialised a parasitic habit feasible 

 at all. For the parasite has evidently become 

 sensitive to the presence of a definite sub- 

 stance which emanates from the host-root. 

 The seed is then stimulated, and it awakes 

 from its dormant condition. It germinates 

 and its roots immediately grow towards, 

 and penetrate, the plant from which it will 

 ultimately draw practically the whole of its 

 food. 



A further state of simplification of vege- 

 tative structure is exhibited by Rafflesia 

 Arnoldii, which is in many respects perhaps the 

 most wonderful of all living flowering plants. 

 It occurs in the Eastern tropics, and it pro- 

 duces the largest flower known, for it may 

 attain to as much as a yard in diameter. 

 The rafflesias are mostly parasitic on vine- 

 like climbers (Cissus), and they pass their 

 vegetative life entirely within the com- 

 paratively slender stems of their hosts. In 

 this stage Rafflesia is extremely simple in 

 structure, and indeed it resembles colourless 

 fungal hyphae more than anything else. 

 The filamentous cells branch through the tis- 

 sues of their host, and it is only when the 

 period of flowering draws near that the para- 

 site gives any sign of what is about to issue 



