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THE INDIVIDUAL ORGANISM 



family. The bacteria inhabit special root nodules formed by the plant 

 (Fig. 33.4) and live at its expense; but the plant also benefits, since 

 through the agency of the bacteria it is able to obtain nitrogen from the 

 air. One more instance of symbiosis may be cited. In many common forest 

 trees root hairs are replaced by fungal filaments; the fungus obtains its 

 food from the tree, while the entry of water into the tree is largely accom- 

 plished by the agency of the fungus. 



No large group of the seed plants has lost the ability to produce chloro- 

 phyll, but here and there among the families we encounter colorless 

 species that have adopted a saprophytic or parasitic mode of life. There 



Fig. 13.17. Dodder, a parasitic vine that lacks chlorophyll. At left, the leafless yellow vine 

 and its flowers; at right, a section of the host stem showing the absorptive organs (haustoria) 

 of the parasite penetrating the host tissues. (Courtesy General Biological Supply House, Inc.) 



are very few saprophytic angiosperms. Well-known examples include the 

 Indian pipe (Fig. 13.16) and the sierran snow plant, both members of the 

 heath family. Parasites are more numerous. Examples are beech drops 

 parasitic on beech roots, broomrape on clover and other roots, squawroot 

 or cancerroot on roots of oaks and other trees, and the dodders (Fig. 

 13.17), yellowish or whitish vines parasitic on herbs and shrubs. All these 

 lack chlorophyll. Such plants as mistletoe combine the parasitic habit 

 with ability to carry on photosynthesis and illustrate the way in which 

 transition to completely parasitic existence was accomplished. 



Finally, we should briefly consider some plants that have developed 

 the ability to capture insects and other small animals as food. These 



