10 ASSOCIATION 



sidered, alternate similarity and dissimilarity of the consti- 

 tuent individuals or species is subordinate as a feature of 

 vegetation only to the primary fact of association. 



In considering the causes which produce association, it is 

 necessary to call in evidence the primary facts of the ontogeny 

 of concrete examples of the working of this principle. These 

 facts are so bound up in the nature of vegetal organisms 

 that they are the veriest axioms. Reproduction gives rise 

 immediately to potential, and ultimately, in the great major- 

 ity of cases, to actual association. The degree and perman- 

 ence 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, haus- 

 toria, holdfasts, rhizoids, roots, etc. The range of immob- 

 ility is very great. In terrestial plants, mobility is confined 

 almost entirely to the period when the individual lies dor- 

 mant 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 repro- 

 duction in an immobile species is to produce association of 

 like individuals, while in the case of a mobile species repro- 

 duction may or may not lead immediately to association. 

 We may lay down the general principle that immobility 

 tends to maintain the association of the individuals of the 

 same generation, i. e., the association of like forms, while 

 mobilty tends to separate the similar individuals of one 

 generation and to bring unlike forms together. With the 

 mobile algae, separation of the members of each generation 

 is the rule, unless the individuals come to be associated in a 

 thallus, or are grouped in contact with the substratum. 

 Flowering plants that are relatively immobile, especially in 

 the seed state, drop their seeds beneath and about the 

 parent plants, and in consequence dense association of the 



