8o INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



substances, are common. These diseases, with the ones produced by 

 attacks of various Fungi, Bacteria, Viruses, and Nematode Worms, 

 and the browsing of lower animals such as Locusts and of higher 

 ones such as Goats, may drastically limit plant distributions. Often 

 the very presence of a plant species in a spot is dependent upon the 

 absence there of serious pests and predators. 



The areas of parasites and saprophytes are naturally limited to 

 ones where suitable hosts or elaborated materials, respectively, are 

 available for attack. Thus, for example, the devastating Late- 

 blight of Potatoes and Tomatoes, caused by the Fungus Phytophthora 

 infestans, is limited to the areas of those crops and of some other 

 members of the family (Solanaceae) to which they belong. Again, 

 the deadly White Pine Blister-rust, Cronartium ribicola, is virtually 

 limited to areas supporting both of the hosts that are necessary for 

 the completion of its life-cycle, namely, five-needled Pines and 

 species of Ribes (Currants and Gooseberries). 



As regards physiological antagonisms due to poisonous residues 

 and excretions, it seems that these may be important in some circum- 

 stances, such as * fairy rings ' and the avoidance by some plants of 

 the shade of certain trees. Thus the roots of Black Walnut {Juglans 

 nigra) have long been known to excrete a toxic substance, juglone, 

 that inhibits the growth of many other plants and can even kill 

 Apple trees. There are also the cases of the western North American 

 members of the Daisy family (Compositae), Partheniuni argentatum 

 and Encelia farinosa, which are known to poison other plants by 

 minute amounts of chemical excretions, thereby reducing competi- 

 tion. Is it possible that this may be one of the factors lying behind 

 the notorious success of this family as colonists ? We do not know, 

 and indeed our information in such fields of study is still only 

 fragmentary. Also undetermined but pregnant with possibilities for 

 research, is the extent to which antibiotic substances may be effective 

 in nature. 



Ecological Limitation 



The realms of physiology range imperceptibly into those of 

 ecology, which in part may be looked upon as the application of 

 physiology to ' field ' conditions. The ecological requirements of 

 different plants are widely various and, as we have seen, commonly 

 limit their geographical areas. This limitation is actually to those 

 regions where appropriate ' habitats ' {i.e. living places) exhibiting 



