]^(;y PLANT STITDTKS 



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

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

 able to grow l)y obtaining nitrogen from the air through the 

 root-tubercles. If tlie 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) Jiif-pliivfs, ctr. — In symbiosis one of the synd)ionts 

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

 become green on account of Algfe which they harbor with- 

 in their bodies (Fig. 152). Like 

 the Lichen-fungus, these ani- 

 mals use the food manufactured 

 by the Algre, wliich in tiirn find 

 a congenial situation for living. 

 By some this would also be re- 

 ^jy^sarjsefc^,^: |fl gardcd as a case of helotism, 



■" *^^ '■■ "''' the animal enslaving the alga. 



/ / I V \ Very definite arrangements 



/' ^/ /«'' are made by certain plants for 



c ^,~-'" harboring ants, which in turn 



guard them against the attack 



i'lc 15:-;, A Ir.'sh-wiitiT jinlyp I//,/- " " 



,/,■«) iitfjiciicd tn;i twig iiiKifeoii- of leaf-cutting insects and oth- 

 in- up.m ,.ig« (C, xviik'h may gj. j'^^.g^ Thcsc plants are called 



1k' scfii tlirougli till.' traiisparoiit i ■ i 



b.Miy wall («!."- (ioLiiBEE.^Eu. Jl i/rm/'rdp/i i/f fs, Avlucli iHcaus 



"ant-pla,nts," or )ni/n)>i'c(ij)I/iloii)< 

 p'(nifs, which means "plants loving ants." These plants 

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

 thorns, or elsewhere^ they provide dwelling places for tribes 

 of u'.irlikc ants ( I''ig. \'>y>). In addition to these dwelling 

 places they provide special kinds of food for the ants. 



(.5) Flowers mul i)isc<is. — A very interesting and iinpor- 

 ta.nt case of symbiosis is that existing between tlowci's ami 

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



