190 BOTANY OF THE LIVING PLANT 



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. 141). 



There may thus be differences in the method of the attack ; but 

 in all of these green parasites the physiological position is the same. 



FIG. 141. 



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

 haustoria penetrating its tissues at intervals. Ceylon, (i Natural size.) 



They establish primarily a relation with the conducting system of the 

 host, and especially with its xylem. Water with its dissolved 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 Photo-Synthesis. 



COMPLETE PARASITES. 



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

 chlorophyll is absent, and they are entirely dependent upon the host 

 for the supply of organic as well as inorganic food. A familiar example 

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

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

 Convolvulus the twining habit (Fig. 142). Clover fields are sometimes 

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

 patches can be seen from a distance by the reddish 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 



