ASSOCIATION 201 



as that of a parasite with its host, which does not constitute vegetation. 

 But it will be seen that the relation of the parasite to the host is practically 

 identical with the relation of the plant to the soil or stratum, and the two 

 concepts mentioned above become merged in such a case. From this it 

 follows that association results in vegetation only when the two ideas are 

 distinct. The concept of association contains a fact that is everywhere 

 significant of vegetation, namely, the likeness or unlikeness of the individ- 

 uals which are associated. In the case of parasite and host, this unlikeness 

 is marked ; in vegetation, all degrees of similarity obtain. As will be evi- 

 dent when the causes which lead to association are considered, alternate 

 similarity and dissimilarity of the constituent individuals or species is subor- 

 dinate as a feature of vegetation only to the primary fact of association. 



Since association contains two distinct, though related, ideas, it is of 

 necessity ambiguous. It is very desirable that this be avoided, in order that 

 each concept may be clearly delimited. For this reason, the act or process 

 of grouping individuals is termed aggregation, while the word association 

 is restricted to the condition or state of being grouped together. In a 

 word, aggregation is functional, association is structural; the one is the 

 result of the other. This distinction makes clear the difference between 

 association in the active and passive sense, and falls in with the need of 

 keeping function and structure in the foreground. 



251. Causes. In considering the causes which produce association, it 

 is necessary to call m evidence the primary facts of the process in concrete 

 examples of this principle. These facts are so bound up in the nature of 

 vegetal organisms that they arc the veriest axioms. Reproduction gives 

 rise immediately to potential, and ultimately, in the great majority of cases, 

 to actual association. The degree and permanence of the association are 

 then determined by the immobility of the individuals as expressed in terms 

 of attachment to each other or to the stratum, such as sheath, thallus, 

 haustoria, holdfasts, rhizoids, roots, etc. The range of immobility is very 

 great. In terrestrial plants, mobility is confined almost entirely to the 

 period when the individual lies dormant in the seed, spore, or propagative 

 part, which is alone mobile. In aquatic spermatophytes, the same is true 

 of all attached forms, while free floating plants such as Lcmna are mobile 

 in a high degree, especially during the vegetative period. Among the 

 algae and hydrophilous fungi, attached forms are mobile only in the spore 

 or propagative condition, while the motile forms of the plancton typify the 

 extreme development of mobility. The immediate result of reproduction 

 in an immobile species is to produce association of like individuals, while in 

 the case of a mobile species reproduction may or may not lead immediately 



