SYMBIOSIS. 



259 



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. 217). Other semi-parasites, such as the 

 mistletoe, attach themselves to the host above ground, and 

 have no true roots of their own. Some parasitic seed plants 

 twine about their hosts, and send 

 into them absorbing organs by means 

 of which they derive all their food 

 from the host. Such is the yellow 

 parasitic vine known as dodder (fig. 

 218, 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. 218, B). 

 Some parasites have the body so 

 reduced that it merely forms a net- 

 work or a hollow cylinder outside 

 the wood of the host and under the 

 bark. From this curious body the 

 few flowers break through the bark 

 and appear upon the surface of the 

 root or stem of the host, quite as though they were a part of 

 it (fig. 219). 



371. Summary. — Plants may live in such relations that 

 one is directly dependent upon the other for its food supply, 

 or they are mutually dependent for food or advantageous 

 conditions. Animals may likewise be directly dependent on 

 plants associated with them. Mutual dependence may exist 

 between plants of the same species, but is commoner between 

 plants of different kinds. One kind may lodge in cavities or 



Fig. 219. — A twig infested with 

 a parasitic seed plant i^Apodan- 

 tkes) whose body is hidden 

 under the barlt of the host, 

 through which a short btanch 

 hearing a few scale leaves and 

 a single flower bursts. Natural 

 size.— After Kemer. 



