SOME ASPECTS OF PERIODICITY IN PLANTS 77 



bough there would be arc-like zones of growth running into one 

 another, just as do arcs of cork in thick bark. Investigation 

 may show that this is the explanation of the peculiar structure 

 of the wood of many tropical species. 



Finally, it may be pointed out that the growth in height and 

 in thickness of the tree as a whole follows a waxing and waning 

 rhythm, so that the successive annual rings traced from the 

 centre at first widen and subsequently narrow until they become 

 approximately constant in thickness. 



The advantages accruing to the plant through certain forms 

 of periodicity are clear, as in the cases of resting spores and 

 seeds ; the annual shedding of leaves or death of epigeous parts ; 

 the early flowering of some trees before foliage obstructs the 

 view to insects or opposes a barrier to the wind; the earlier 

 unfolding of foliage of some plants before these are overshadowed 

 by rivals ; the differences in the times of leafing, flowering and 

 fruiting in various species, which thus avoid contemporaneous 

 competition for the various chemical elements ; the rapid upright 

 growth of young light-demanding trees contrasting with the 

 slow, often more spreading, growth of young shade-enduring 

 species. But in other cases the benefit to the plant of certain 

 forms of periodicity is unknown or possibly non-existent, in the 

 latter case being due perhaps to inevitable correlation or to the 

 bonds imposed by ancestry. 



