934 MINNESOTA BOTANICAL STUDIES. 



to have no effect upon the life-history and development of the 

 symbionts. 



There is no doubt that accidental symbioses forms the basis 

 whence gradually emerged all other forms of symbiosis. Per- 

 haps few symbiotic associations were from the very first mark- 

 edly antagonistic, and still less rarely markedly mutualistic for 

 reasons already stated. 



To the category of accidental symbiosis also belong the as- 

 sociation of climbing plants and their living supports. The 

 symbiotic relationship was at first merely accidental resulting 

 from the contact. It is a striking example illustrating how 

 marked and highly specialized morphological adaptations fav- 

 oring one function may initiate widely different morpho physio- 

 logical changes. Iu the case of climbing plants it is impossible 

 to know when the symbiotic relationship begins to overbalance 

 the function of mechanical support. It is just as difficult to 

 determine when marked symbiotic phenomena begin to mani- 

 fest themselves. It is safe to conclude, however, that the 

 morphological changes favoring climbing and support pro- 

 gressed considerably before any marked symbiotic relation- 

 ships occurred. 



It is also evident that accidental symbiosis is a condition 

 readily subject to change, since the permanency of symbioses 

 is in direct proportion to the degree of mutualistic specializa- 

 tion. Each plant and animal may enter into accidental sym- 

 biotism with other plants and animals. In a given animal this 

 association changes with a change of locality, in temperature, 

 or of moisture; in fact, with every change in the environment. 

 The absence of all permanency in morphological and func- 

 tional relationship characterizes accidental symbiosis. It re- 

 sembles a form of haphazard experimentation on the part of 

 nature to determine whether or not a definite symbiotic rela- 

 tionship can be established. 



2. Contingent Symbiosis. — In this form of symbiosis the re- 

 lationship of the organisms is already sufficiently marked to 

 give the semblance of an elective affinity, although the func- 

 tional interdependence is as yet not manifest. It is of wide 

 occurrence among widely different organisms. Man}'" phenom- 

 ena heretofore recognized or variously classified as parasitism, 

 perhaps belong to this category. Most of the phenomena rec- 

 ognized by the German scientists as Raumparasitismus also 

 belong here. The citation of a few examples will suffice to ex- 

 plain the nature of contingent symbiosis, and to distinguish it 



