PERIODICITY IN DEVELOPMENT. I 347 



More exact investigation shows us that the differences between spring plants 

 and trees are by no means deep-seated. The summer rest of the former may- 

 be readily compared with the absence of leaf-unfolding in summer in all spas- 

 modically-budding trees ; the foliage leaves of such plants die off much sooner 

 than in the case of trees, but it can scarcely be doubted that embryonic growth 

 continues during the whole summer in the subterranean organs. Nor is the early 

 commencement of vigorous growth in the buds in autumn so very extraordinary, 

 since, according to Askenasy, the buds of trees do not cease developing during 

 the winter. The characteristic feature of spring plants lies only in this, that their 

 periods are somewhat moved forward and that their leaves are very short lived. 

 They also exhibit a resistance at certain times to artificial forcing ; thus tulips 

 and hyacinths cannot be made to flower before December. In all probability the 

 formation of shoots is more readily possible in such plants just before the beginning 

 of the resting period ; at least certain results obtained by Schmid (1901) may 

 be interpreted in this sense. This author has also shown that the special resting 

 period in the case of the potato may be shortened by external causes. 



The observations which have been quoted, as well as the fact that members 

 of one and the same plant are in a state of rest at different times, teach us that 

 under given external conditions only individual functions come to rest or 

 exhibit reduced activity, and that these resting periods cannot be directly due 

 to unfavourable external factors. This may be proved even more readily in the 

 case of many tropical plants which show periodic growth even though external 

 conditions be constant and favourable. We are indebted to Schimper (1898) for 

 exhaustive studies on this subject. According to his researches a large number 

 of trees exist at Buitenzorg in Java which at longer or shorter intervals, once 

 to six times annually, throw off all their foliage and, often after only a few days, 

 produce fresh foliage once more. Other plants behave even more remarkably, 

 e.g.Amherstia nobilis, in which the individual branches act independently of each 

 other, so that we may find on a single tree at the same time branches with ter- 

 minal resting-buds and shoots in all stages of development. Trees transplanted 

 from a temperate to a tropical climate behave just like Amherstia, inasmuch as 

 they lose, not periodicity itself, but its relation to seasons. The appearance of 

 Magnolia ynlan in December and January in the Hill station at Tjibodas near 

 Buitenzorg is described by Schimper (1898, 266) in the following terms : 

 ' Individual branches bare of leaves but with leaf and, here and there, flower 

 buds ; others with young leaves and full blown flowers ; others with adult 

 leathery leaves and withered flowers ; others with leaves with autumn colouring 

 readily deciduous.' 



In other tropical climates, always warm and moist, such as the Brazilian 

 rain forest near Para, we find examples of periodicit3^, e.g. Hevea hraziliensis. 

 According to the observations of Huber (1898) the fiill-grown tree frequently 

 produces only one or often two shoots in the year after the leaves have 

 more or less completely fallen off. The young trees behave in an especially 

 interesting manner. One of these formed, during the rainy period of 1896-7, 

 five shoots, each of which was in full leaf in thirty days, and then remained 

 quiescent for about ten days. Each shoot had at first short internodes, then 

 longer, and then finally shorter internodes once more. It bore scale-leaves at 

 the base, then foliage-leaves, and terminated in a bud enclosed in scales destined 

 to be the next shoot. The five shoots opened out on Dec. 10, 1896. Jan. 20, 

 March 12, April 25, and June 6, 1897, respectively ; the succeeding shoots were not 

 observed exactly, but it was established that three had been formed during 

 the remainder of 1897 and three more in the first half of 1898. Since, however, 

 different examples are always in different phases of development, it cannot be 

 doubted that the periodicity is due entirely to internal causes. As the plant 

 gets older the number of shoots decreases, and, finally, as already noted, only one 

 appears each year. This rhythmic alternation of activity and quiescence which 



