90 



PLANT STKUCTUEES 



nary crops have exhausted the soil of its nitrogen-contain- 

 ing salts, and it has become comparatively sterile, clover is 

 able to grow by obtaining nitrogen from the air through the 

 root-tubercles. If the crop of clover be " plowed under," 

 nitrogen-containing materials which the clover has organ- 

 ized will be contributed to the soil, which is thus restored 

 to a condition which will support the ordinary crops again. 

 This indicates the significance of a very ordinary " rotation 

 of crops." 



(4) Ant-plants, etc. — In symbiosis one of the symbionts 

 may be an animal. Certain fresh-water polyps and sponges 

 become green on account of Algse which they harbor with- 

 in their bodies (Fig. 79). Like 

 the Lichen -fungus, these ani- 

 mals use the food manufactured 

 by the Algs, which in turn find 

 a congenial situation for living. 

 By some this would also be re- 

 garded as a case of helotism, 

 the animal enslaving the alga. 



Very definite arrangements 

 are made by certain plants for 

 harboring ants, which in turn 



'iG. 79. A fresh-water polyp {Hy- 

 dra) attached to a twig and feed- 

 ing upon algffl (6'), which may 

 be seen through the transparent 

 body wall (7^).~(,'aldwell. 



guard them against the attack 



of leaf-cutting insects and oth- 

 er foes. These plants are called 

 Myrmecnpliytes, which means 

 "ant-plants," or myrmeccpJiilous 

 plants, which means " plants loving ants." These plants 

 are mainly in the tropics, and in stem cavities, in hollow 

 thorns, or elsewhere, they provide dwelling places for tribes 

 of warlike ants (Fig. 80). In addition to these dwelling 

 places they provide special kinds of food for the ants. 



(5) Flowers and insects. — A very interesting and impor- 

 tant case of symbiosis is that existing between flowers and 

 insects. The flowers furnish food to the insects, and the 



