IRREGULAR NUTRITION 223 



a sucker penetrates the living tissues of the host. In Viscum a 

 suctorial system spreads from the original centre within the tissues 

 of the host, penetrating along the region of the cambium. But in 

 Loranthus the shoot of the parasite creeps along the outside of the 

 host, and puts in suckers at intervals (Fig. 150). 



All these green parasites establish a relation with the conducting sys- 

 tem of the host, especially with its xylem. Water with its dissolved 



Fig. 150. 



Loranthus parasitic externally upon a branch of an Alligator Pear, by means of 

 haustoria penetrating its tissues at intervals. Ceylon. (\ natural size.) 



salts is then drawn off from the transpiration stream. In the root- 

 parasites this supply is additional to what they can themselves absorb. 

 But in those attached to the shoot their whole supply is thus obtained. 

 It is uncertain whether or in what degree organic supplies may also be 

 abstracted. In any case the presence of chlorophyll shows that these 

 green parasites are not wholly dependent upon their host, but can 

 themselves carry on photosynthesis. Some at least of the root-para- 

 sites can grow quite well without the parasitic connection, though this 

 is naturally impossible in shoot-parasites such as Mistletoe, where 

 normal roots are absent. 



Complete Parasites. 



In these, though the plant may show various colours, the green of 

 chlorophyll is almost absent, and it is evident that the parasite leads a 

 heterotrophic existence at the expense of the host. A familiar example 

 is the Dodder (Cascuta), a genus represented in the British Flora by 

 three species. It belongs to the Convolvulaceae, and shares with 

 Convolvulus the twining habit (Fig. 151). Clover fields are sometimes 

 attacked by one of these Dodders (Cuscuta trifolii), and the infected 

 patches can be seen from a distance by the reddisli colour of the 

 parasite and the stunted growth of the clover upon which it preys. 

 Examination shows the Dodder to have cylindrical stems, which twine 



