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 
ef 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 in 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 Lemna 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 
