HAUSTORIAL ORGANS OF PARASITES 



253 



the Mistletoe and certain other Loranthaceae, the whole aim of the 

 parasite is to effect a connection with the water-conducting system of the 

 host. Those hemi-parasites which are rooted in the ground often develop 

 typical root-hairs, in this respect resembling seedlings which have formed 

 a root system, but which still draw upon the store of plastic material 

 deposited in the seed. Where a parasite is entirely devoid of chlorophyll, 

 its haustorial organs frequently develop pencil-shaped or filamentous 

 processes, in accordance with the principle of maximum exposure of 



Fig. 99. 



Rhizoids of saprophytic mosses. A. A rhizoid of Eurhynchium praelongum, growing 

 in the epidermis of a decaying beech-leaf and producing lobed haustorial processes. 

 B. Perforation of the wall of a Coniferous tracheide by a rhizoid of Webera nutans. 



surface. In such cases the absorbing tissue takes the form of 

 hypha-like strands of cells, which penetrate the tissues of the host in 

 various directions (Lathraea squamaria, Cusci/ta, robanche). This type 

 of absorbing tissue not infrequently displays a very considerable 

 division of labour. Tracheidal cells, which are continuous with the 

 water-conducting system of the parasite, strive to place themselves in 

 contact with the hadrome elements of the host ; sieve-tube-like 

 haustorial cells exploit the protein-conducting leptome, while the less 

 specialised absorbing strands withdraw non-nitrogenous plastic materials 

 from the medullary and cortical parenchyma {Orobanche). The most 

 far-reaching modification in connection with a parasitic mode of life is 

 seen in the Rafflesiaceae, where the vegetative body is reduced to a 

 mycelium -like system of absorbing strands. 



After these general remarks, a few of the cases which have been 



