236 THE BIOLOGY OF FLOWERING PLANTS 



function as absorbing organs. From these discs suckers 

 are sent down into the host tissue, which is dissolved away 

 by secreted enzymes. The suckers spread out inside the 

 host ; vascular tissue is formed and connects up with the 

 wood and bast ; prosenchymatous filaments penetrate the 

 cortex and even the pith, and tap living cells. 



The nature of these suckers is not clear. Some botanists 

 regard them as modified adventitious roots, and in some 

 species, e.g.Cusciita europcea,they are produced endogenously. 



After connection has been made with a host growth is 

 very vigorous. The stem passes from shoot to shoot, 

 branching freely ; the parasite may completely exhaust and 

 smother the host plant, and may do extensive damage to 

 crops of clover and flax. The little bunches of flowers are 

 produced abundantly. 



It is interesting to note that in Cuscuta and Cassytha 

 reduction runs on lines different from those common in 

 other advanced parasites. The foUage indeed is gone, but 

 the stem is well developed ; this is of course to be related 

 to their unique mode of attachment to the host, and brings 

 with it the advantage of rapid spread over available host 

 plants. Alone among the parasites, Cuscuta can be grown 

 independently of host plants in a sugar solution. In such 

 cultures Cuscuta monogyna flourishes, and produces flowers 

 and fruit (Molliard, 1908). 



General Considerations. — Reviewing this account of 

 parasitic flowering plants, we see that, while we are well 

 informed as to the structural relations between them and 

 their hosts, little of the physiological relation is known. We 

 do not even know exactly whether green parasites receive 

 only water and inorganic salts. As to the forms in which 

 organic compounds are absorbed by complete parasites 

 we know nothing. Nor do we know much of the factors 

 which bind a parasite to its particular hosts. It may be 

 surmised that some plants are unsuitable as hosts because 

 they offer difficulty to penetration, but this explains little. 

 In Cuscuta enzymes play a part in securing penetration ; 

 to what extent they are active in other cases is unknown. 



