survive in nature because suitable habitats are usually not avail- 

 able, unless the natural vegetation has been disturbed.^ Natural 

 hybridization between species in the same genus, and even be- 

 tween species in different genera, is more common in the grass 

 family than in most families of flowering plants, probably because 

 grasses usually grow close together, are wind-pollinated, and 

 many kinds are self-incompatible. ^^^ Parents differing consider- 

 ably in requirements and amplitude are likely to produce hybrids 

 which possess broad adaptation. In artificial breeding it is 

 important that both parents have ranges of requirements and 

 amplitude which approach the environmental conditions of the 

 area where the hybrid is to be grown, for example, species 

 adapted to the southwestern part of the Great Plains, where 

 winters are cold and dry and the summers hot with frequent 

 showers in late summer, are not suitable as parents of hybrids to 

 be used in parts of California with cool, rainy winters and hot, 

 dry summers.^° It has been found, however, that the genetic sys- 

 tems of some wild species can be rearranged through interspecific 

 crossings so that the modified amplitude of the offspring enables 

 them to live in various habitats. ^^ 



RELATIONS AMONG INDIVIDUALS OF THE SAME 

 OR DIFFERENT SPECIES 



The complexity in the interrelations of plants is caused by the 

 number, variety, and frequency of interactions between many 

 individuals. The interactions change with the time of day and 

 with the season; some are favorable for growth and reproduction, 

 some are unfavorable, and some appear inconsequential. These 

 interrelations have been grouped under various terms such as 

 symbiosis in a broad sense, ^^^'^^ coaction,^^ and types of reactions 

 or interactions. ^^^ A useful classification of interrelations on the 

 basis of contiguity is as follows: 



( 1 ) Interrelations of organisms that are not in actual contact 

 or not in contact continuously; disjunctive. 



(2) Interrelations of organisms that are in close bodily con- 

 tact, where, if this contact is broken, one or the other 

 organism suffers; conjunctive. 



26 • Species amd Popimlatioxms 



