SOME ASPECTS OF PERIODICITY IN PLANTS 63 



The induction of a special rhythm by intermittent geotropic and 

 heliotropic stimuli was accomplished by F. Darwin and Miss 

 D. Pertz. In highly organised plants the difficulties of inter- 

 preting periodic action are great because, on the one hand, if 

 the rhythm be inherent it may be innate in the protoplasm 

 itself or may arise from the general architecture and consequent 

 limitation in mode of action or may result from both causes ; 

 whilst, on the other hand, if the rhythm be induced the diffi- 

 culty in analysing the phenomenon is proportionate to the 

 complexity of the plant and the ever-changing environment. 



The lengths of the periods under discussion vary from a 

 fraction of a second, in a moving cilium, to days, months or 

 years, as represented by the longevity of a cell or organism. 



The subject of longevity itself offers many problems for 

 solution, even along paths that are strewn with facts already 

 known. Very familiar is the fact that in many species of plants, 

 varying from simple to the most complex, death is associated 

 with a special form of reproduction. This is so much the case 

 with phanerogams that if individuals of monocarpous species be 

 prevented from flowering, their length of life may be prolonged 

 apparently indefinitely (?) ; it is thus that the Chinese preserve 

 the lives of monocarpous bamboos. Between polycarpous and 

 monocarpous species various transitions occur. It is a familiar 

 fact that our ordinary trees have seed-years succeeded by 

 several more or less barren ones, during which, according to 

 Hartig, reserve material is accumulating for the next effort. 

 Here the explanation given of seeding is that it is postponed 

 until a certain minimum of starch and other food-material is 

 stored. Many shrubs and certain Indian bamboos have branches 

 that inevitably die after fruiting. If all the branches of the 

 bamboo thus flower and die to the ground, the plant is so 

 weakened or depleted of food that in the following season it 

 either sends up slender seeding-like shoots or is exterminated. 

 Such species lead on to monocarpous types, if such absolutely 

 monocarpous bamboos do actually exist. The cause of death 

 in polycarpous plants is quite unknown, indeed it is impossible 

 to say whether or no a tree ever dies of old age. It is certain 

 that isolated plant cells and organs, for example, sexual cells, 

 pollen-grains and seeds, perish naturally, that is to say, 

 unattacked by parasitic foes. But the lives of cells (pollen 

 grains) and organs (seeds) may be prolonged by appropriate 



