SOME ASPECTS OF PERIODICITY IN PLANTS 69 



the temperature has reached its maximum. The inherent 

 nature of this type of rhythm is demonstrated by the following 

 facts : 



1. These features present themselves in various land-plants 

 developing at different times, growing in full sunlight or in 

 shade, as well as in submerged plants ; they are seen not only 

 in cold-temperate trees but also, as Raciborski showed, in 

 trees growing in perennially moist tropical forests, where 

 there is neither a dry nor a cold season to interrupt vegetative 

 activity. 



2. On different individuals of the same species, and on 

 different shoots of the same individual, both in cold-temperate 

 and in moist tropical regions, the various shoots may entirely 

 disagree as to their phase of development. In fact in the latter 

 regions, as Schimper pointed out, a single tree may at one and 

 the same moment have branches showing respectively resting 

 buds, actively sprouting buds, flowers and fruits, thus recalling 

 the conditions during winter, spring, summer and autumn in 

 cold-temperate regions. 



The greater length of the intermediate internodes is not 

 due to the production of larger leaves on the same axis, for 

 Miss Tinne Tammes has shown that removal of leaves from the 

 stem does not destroy the periodic procession of internodes 

 of differing lengths. It may also be noted that the changes of 

 dimensions in leaves and internodes of a shoot, often at least, 

 do not synchronise in phase. 



Clearer insight into the nature of the longitudinal symmetry 

 resulting from this periodicity of the internode-lengths may 

 be gained by the construction of curves in which successive 

 ordinates represent the lengths of the successive internodes. 

 The resultant curve — the internode-curve — is often more or 

 less characteristic of the species and therefore of diagnostic 

 value, though it is subject to modification by external factors. 

 The internode-curves of the axes of some herbaceous species 

 are not more or less smooth and regular but are marked by 

 zig-zags. In the investigated cases I have shown that these 

 deviations, though to some extent due to variations in external 

 factors, are largely inherent and due to longitudinal displace- 

 ment of some of the leaves, as for instance when the phyllotaxis 

 has changed from the cyclic to acyclic type. Very little work 

 has been done on this subject ; apparently irregularity in the 



