CHAPTER V — REPRODUCTION AND DISPERSAL 



I. REPRODUCTIVE BEHAVIOR IN THE SEEDLESS PLANTS 



General remarks. — The process by which organisms give rise to 

 others of their kind is known as reproduction. The essential element 

 in reproduction is the organization of a cell or a group of cells, which, 

 if detached, possesses a capacity for independent development, and hence 

 may be called offspring. Closely associated with reproduction is dis- 

 persal, which makes possible the development of organisms in new 

 territory, and without which reproduction would be of small significance.' 

 Detachable structures, however produced, if capable of dispersal, may 

 be called disseminules, and it is obvious that upon such capacity for de- 

 tachability and for subsequent mobility, the effectiveness of dispersal 

 and therefore the success of a species must largely depend. 



Most plants give rise to many new individuals within their lifetime, 

 but only a few of these come to maturity and have progeny. The vast 

 majority of plant disseminules fail to lodge in places suitable for de- 

 velopment, while of those that make a start, but a very few ever reach 

 maturity. The preemption of space by other plants, the submergence 

 of the weaker individuals, and untoward physical conditions cause the 

 destruction of most plant offspring and prevent the otherwise rapid 

 advance of any given species over the face of the earth. Three kinds 

 of reproduction may be distinguished, each with its characteristic dis- 

 seminules, namely, vegetative reproduction or propagation, reproduction 

 by asexually formed spores, and sexual reproduction. 



Vegetative reproduction. — General characteristics. — In vegetative 

 reproduction or propagation, new plants are formed in connection with 

 the vegetative organs, and the offshoots, sometimes known as propa- 

 gules, more or less resemble the parts from which they issue. Vegetative 

 reproduction is associated with periods of activity, while other forms of 

 reproduction commonly terminate such periods. Vegetative dissemi- 



^ Dispersal without reproduction, though a conspicuous feature in animals, is relatively 

 rare in plants; however, it is illustrated by certain motile algae (as Chlamydomonas and 

 Volvox, figs. 21-29) ^"*i bacteria (figs. Z4-20), and also by the amoeboid movements of 

 myxomycete plasmodia (fig. 3). 



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