34o 



PLANT LIFE. 



culosis, are known to be due to the transfer of the parasite 

 from the diseased individual to' the healthy one. In a similar 

 way bacteria live as parasites upon green plants, causing 

 disease and often death. The number of bacterial diseases 

 among plants is relatively small, for comparatively few bacteria 

 have been able to adapt themselves to living in the acid cell- 

 sap of plants. The number of diseases of plants due to 

 parasitic fungi, on the contrary, is very large. (For the mode 

 by which parasitic fungi gain entrance to the bodies of their 

 hosts, see ^f 52.) 



465. 2. Seed plants. — A few seed plants have adapted 

 themselves to a parasitic life upon others. Some may be 

 reckoned as semi-parasitic, having 

 still green leaves and true roots. 

 In addition, however, special organs 

 are developed for attaching the 

 parasite to the roots of other plants, 

 from which at least a water supply 

 and probably food materials are 

 absorbed (fig. 379). Other semi- 

 parasites, such as the mistletoe, at- 

 tach themselves to the host above 

 ground, and have no true roots of 

 their own. Some parasitic seed 

 plants twine about their hosts, into 

 which they send absorbing organs 

 by means of which they derive all 

 their food from the host. Such is 

 the yellow parasitic vine, known as 

 dodder (fig. 380, A ) . These plants 

 germinate in the ground, and as seedlings possess true roots, 

 but after attaching themselves to the host the lower part 

 of the stem dies away so that the true roots are transient (fig. 

 380, B). Some root parasites begin to germinate upon the 



Fig. 381.— A twig infested with a 

 parasitic seed plant (Afiodan- 

 thes) whose body is hidden un- 

 der the bark of the host, through 

 which a short branch bearing a 

 few scale leaves and a single 

 flower bursts. Natural size. — 

 After Kerner. 



