64 PARASITES AND SAPROPHYTES [CHAP. 



such as dodder, to be next described, in which 

 the irritation acts more minutely on the deeper- 

 seated cortical tissues, wherein is formed the 

 absorbing organ, while in the Euphrasiece, as 

 we have seen, the suckers are formed by the 

 epidermis alone. 



The method by which an aerial leafless parasite 

 fixes itself to a host-plant is well illustrated by 

 the dodder. The seed of this plant has no 

 cotyledons and resembles a minute coiled thread. 

 It germinates by fixing its club-shaped root-end 

 in the soil, and draws up water by means of root- 

 hairs ; it then sends up a thread-like stem, which 

 " circumnutates " like a tendril in search of a 

 support. As soon as it comes in contact with a 

 suitable host-plant, it quickly forms an adhesive 

 " haustorium," x and from that moment the lower 

 part dies and the dodder becomes entirely para- 

 sitic and grows rapidly all over the host-plant. 

 If the seedling fails to find a host its stem falls 

 to the ground, and provided the soil be moist, 

 the apex will grow while the root-end dies. The 

 nourishment is then continually being transferred 

 from one end to the other, as the stem, as well as 

 the root-end, dies from below upwards. The 

 little plant thus " moves " along, as it were, by 

 growth until it may come in contact with some 

 suitable plant. If it fail to do so, it will of 

 course perish in time. In circumnutating it 



i The analogous process in the climbing pads of the Virginia 

 creeper is very suggestive (see p. 10). 



