Chap. II] PERIODIC PHENOMENA IN TEMPERATE ZONES 433 



are externally visible fall into three periods : (a) a period of growth of the 

 foliage-buds (April-May), (b) a period of assimilation during which axes 

 and roots grow in thickness 1 and the winter buds are formed (May- 

 September), and (c) a period of retardation and decline terminating in 

 the autumnal leaf-fall. Amongst all these phenomena, that of the 

 development of buds, especially of the flower-buds, has proved best 

 adapted for the study of the periodic phenomena in their separate 

 details. 



According to Askenasy, who was the first to conduct a thorough 

 investigation into these matters, the development of the flower-buds of 

 the gean-tree is distributed over two periods, which are separated by 

 a period of rest, or rather of extremely slow growth. The period of 

 rest, at Heidelberg, lasts from about the end of October to the beginning 

 of February, that is to say about three and a half months; it is therefore 

 considerably shorter than the period included between the more striking 

 features of defoliation and sprouting of the buds that is usually described 

 as the period of rest. Next year's foliage-buds are already laid down 

 at the flowering time ; those of the flowers in the course of July. 



The growth of the buds during the first growing period, i. e. during 

 the summer and early autumn, is very slow and uniform up to the 

 commencement of the period of rest, when it becomes almost nil. 



At the beginning of the second or spring period of growth progress 

 is at first still slow, but becomes gradually more rapid, and finally so 

 rapid that, towards the end of their development, the flower-buds in 

 6-10 days increase to double and treble their fresh weight. Growth 

 is constantly accelerated till shortly before maturity, but is retarded 

 immediately before its close. The whole development therefore affords 

 a splendid example of Sachs' grand period of growth. 



In the increase in size of the buds water naturally takes a greater 

 share than does dry material. Of the total weight of freshly blown 

 buds I may be placed to the credit of spring growth, | to that of summer 

 growth. The dry weights however are in the ratio of f : £. A hundred 

 buds during the spring period increase in dry weight by 6 grammes. If 

 the tree possesses 200,000 flower-buds — mostly too low an estimate — for 

 their formation 12 kilos, of dry material are necessary. 



The curve of growth of the gean-flower, characterized as it is by a 

 sudden sharp ascent, is only occasionally, and then merely to a certain 

 extent, dependent on temperature. Any influence of temperature is not 

 discernible during the summer period, and even during the spring period 

 variations of temperature cannot alter the course of the curve of groivtJi. 

 In other words, if the temperature in February be higher than in March. 



1 According to von Mohl, roots continue their circumferential growth, however slowly, 

 during winter. 



SCHIMIER P f 



