662 General and Applied Biology 



Insects not only transmit plant diseases but as a result of their chewing, 

 sucking, boring, and egg-laying activities are responsible for many serious 

 consequences in the tissues of higher plants. Enlargements known as 

 galls on leaves, stems, etc., are common examples. Bark lice, mealy bugs, 

 various scale insects, etc., cause a great variety of serious diseases in 

 plants. 



The various causes and effects of the different diseases of plants and 

 animals cannot be discussed in detail here, but the reader is referred 

 to other sources for those in \vhich there is a particular interest. 



II. SYMBIOSIS 



In some instances, in the living world there is more than mere living 

 together in harmony, for there is more or less of a mutual helpfulness 

 between certain living organisms. A condition in which two species of 

 organisms (known as symhionts) live together with mutual benefit to 

 both is known as symbiosis (simbi -o' sis) (Gr. sym, together; hios, life). 

 In some cases this association is so complete that there is organic unity 

 in which each type of organism contributes something to the other with 

 which it is living. In the so-called green Hydra there live small green 

 algae (plants) which photosynthesize food by combining water and car- 

 bon dioxide, the latter being given off by the Hydra. The foods and the 

 by-product (oxygen) of photosynthesis may be used by the Hydra. In 

 a group of plants known as lichens {W ken) (Gr. leichen, liverwort) 

 (Fig. 327) there is a close relationship between the green, chlorophyll- 

 bearing algae and the nonchlorophyll-bearing fungi of which lichens are 

 composed. The algae supply foods for the fungi, while the latter give 

 protection, supply water, etc. 



Termites feed on cellulose of wood but are unable to digest it. Certain 

 flagellated protozoa within their intestine render the cellulose digestible 

 for both. These protozoa cannot exist outside the termite intestine. Cer- 

 tain types of ants protect certain species of aphids (plant lice) and in 

 return use as food the sweet "honey milk" produced by the latter. The 

 so-called green paramecium {P. bursaria) has within its endoplasm the 

 unicellular, green alga (Chlorella vulgaris). The alga uses the wastes 

 of the Paramecium and gives food and oxygen in return. Certain sponges 

 and green algae possess a relationship similar to that described above. 



The Portuguese man-of-war, a coelenterate, possesses long tentacles 

 among which live certain species of fish. The nematocysts of the tentacles 

 protect the fish, and the latter share some of the foods which they cap- 

 ture with the Portuguese man-of-war. 



