SOME ASPECTS OF PERIODICITY IN 



PLANTS 



By PERCY GROOM 



The successive morpholigical changes that collectively constitute 

 the ontogeny of an individual exemplify but one of the many 

 manifestations in protoplasm of periodic action, protoplasmic 

 periodicity being often revealed in rhythmic physiological 

 activity almost or entirely unaccompanied by appreciable change 

 in structure. Clear cases of such periodicity are afforded by 

 unicellular organisms, in which various forms of functional 

 activity wax and wane in due order : for instance, beginning 

 with rhythmic contractility leading to locomotion associated with 

 irritability that constrains the organism to move in a definite 

 direction ; then passing on to special secretory activity that 

 brings into being a cell-wall, whose formation may be succeeded 

 by obvious growth and rapid photosynthesis, which in turn 

 may give way to special kinds of contractility or secretory 

 activity coincident with the assumption of a reproductive or 

 resting phase. During these more or less obvious phases there 

 is often, at least, invisible rhythmic change of such a nature that 

 one and the same stimulus is succeeded by responses that differ 

 with the age of the cell : such changed behaviour in response to 

 stimuli of light and gravitation are especially familiar among 

 naked mobile masses of protoplasm of Algae and Myxomy- 

 cetes. To what extent this rhythmic functional activity of the 

 protoplasm may ultimately be expressed in physico-chemical 

 terms as simple as those explanatory of the rhythm of inter- 

 acting chemical bodies or of enzyme-action, it is impossible to 

 say. Rhythmic activity in the simple plant or cell may be 

 autonomic or induced and is subject to modification by external 

 factors. Klebs' work on the induction of reproduction in Algae 

 and Fungi especially exemplifies such change in the length of 

 some of the phases constituting a complete rhythmic cycle; 

 whilst the ether treatment and other methods of forcing, also the 

 changed phenology of plants transferred to different altitudes or 

 latitudes, illustrate analogous changes induced in Higher plants. 



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