THE INTERDEPENDENCE OF LIVING THINGS 



57 



causes disease. In some instances, the complicated life history is so 

 bound up with more than one host that if one of the hosts is absent 

 a hnk in the chain of life is broken, the life cycle cannot be completed' 

 and the parasite dies. The black-stem grain rust, which ref,uires 



^^ 



recC spore 

 blov/n to 

 anotber^ stem 



recC or 

 Sxtmreierc 

 rixst on 

 •wheat stem 



barlserrv rtcst 

 spore in?ectintf 

 ths cells of -^heat 

 stem ira 

 spring- 



"bocrbei^ry 

 leaves 



mfscts stem 



through 



breo-^hing" 



?«""«. red rust syjrecuil^ , 

 from stem to stem/ 

 cCixriijg' Sixmme'P 



blaclc or 

 ^vinter rust 

 lives on 



straw thrcuflh , 

 winter- * " 



infection form', 

 barberry rust 

 onborberrjlea.^ 

 a cluster cup' 



The life history of black stem grain rust. 



controlled. 



r, ■!, black spora 



inject ing^ 

 bocfics, sporicCa 



a spoT~id.ium 



infects the 



Cells of a 



barberry leaf 



Explain how this rust may he 



both the barberry plant and the wheat to complete its life history ; 

 the pine tree blister, which lives on the currant or gooseberry at one 

 stage of its life history, and on the pine at another ; and the parasite 

 causing malaria, which requires both the anopheline mosquito and the 

 blood of man to complete its cycle, are examples. 



The Chemical Relationship of Plants and Animals 



The study of plant and animal ecology may be said to be analogous 

 to the study of human economics. Social conditions among men, 

 animals, and plants are all determined by the environmental factors 

 present, but chiefly by the availability and abundance of food. The 

 world's food supply in the long run depends upon the chemical ele- 

 ments making up the environment and energy derived from the sun. 



