SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 137 

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tion and occasional exposure to air. The greater keeping power of ripe 

 fruit, as compared with green fruit of the same kind, is no doubt due to 

 the fact that the ripe fruit contains comparatively more sphserocytes 

 than does the green fruit. The further study of these highly interesting 

 structures in the plant kingdom, and their analogues in the animal king- 

 dom, will no doubt reveal new chapters in the study of cytology and also 

 in immunology. 



The entire discussion of cytology, of the relationship of the cells (so- 

 matic as well as germatic) of living organisms, unicellular as well multicel- 

 lular, etc., is intimately associated with the problems of symbiosis in the 

 broader sense, as has been indicated in the preceding. We shall now take 

 up the more specific forms of symbioses, generally recognized as such. 



2. The Phenomena of Symbiosis 



Introduction. All living organisms manifest a more or less intimate 

 biological interdependence and relationship. In fact, their very existence 

 depends upon this condition; therefore no organism, no matter how 

 simple or how complex its structure may be, is the result of a wholly inde- 

 pendent phylogenetic development. Upon careful study and investiga- 

 tion it is found that, although this interrelation and interdependence 

 varies greatly as to quality and quantity, there may be found innumerable 

 intermediary phenomena which make it difficult to draw the dividing lines. 

 Such a difficulty is, for instance, encountered in attempting to distinguish 

 between mere "associations" or societies (according to Warming and 

 others) and true symbiosis. Both are evident phenomena of biological 

 interdependence with the general difference that in the former the inter- 

 dependence is remote, in the latter more close. 



Great difficulty is encountered in limiting and defining the biological 

 relationships in the animal kingdom. Highly automobile organisms do 

 not permit the ready establishment of symbiotic relationships as we have 

 come to understand them. Symbiosis presupposes a certain relative 

 fixedness of the organisms. We may find clearly defined symbioses be- 

 tween highly automobile organisms and those which are comparatively 

 non-motile. Here it is very essential to keep distinct the difference 

 between auto-mobility and passive motility (immobility). The former 

 tends to counteract or reduce the occurrence of symbiosis; the latter 

 favors its occurrence as well as its adaptive modification, as will be ex- 

 plained later in the discussion. The most clearly defined and most highly 

 specialized forms of symbiosis occur between non-motile organisms. 



Motility or non-mo tility of organisms has little or no direct influence 

 upon the more remote biological relationships. From the fact that 

 these latter phenomena are most conveniently limited, geographically, 



