68 SCIENCE PROGRESS 



cence-buds in the normal manner so that these are visible in 

 winter, but it does not adopt the normal habit of opening them 

 before the leaves appear : in this species, blossoming is further 

 postponed until the leaves unfold at any time between spring 

 and midsummer. At the opposite extreme are three other 

 species, namely, another American alder, A. marituna, which 

 is more southern as it grows in Southern Delaware, Maryland 

 and the Indian Territory ; also two East Indian species, 

 flourishing in a sub-tropical or tropical climate, A. nepalensis 

 (descending to 3,000 feet on the Himalayas, and 6,000 feet in 

 Upper Burma) and A. nitida (descending from the Himalayas 

 on to the plains). In these three species, blossoming is hastened 

 in comparison with other species of Alnus, for the inflorescences 

 open in autumn as branches of the current year's shoot. The 

 fact that Alnus is the betulaceous genus including species which 

 show the fagaceous periodicity is of interest as supporting the 

 view that the Betulaceae are reduced Fagales living in a climate 

 colder than the ancestral one, for Alnus is at the same time the 

 genus having the most complete flowers as regards perianth, as 

 well as undivided stamens, and thus showing the nearest ap- 

 proach to the typical fagaceous flowers of Quercus and Castanea. 



The explanation of the early flowering of the Common ash 

 and of the Elms and of the insertion of their inflorescences on 

 the previous year's twig, is probably the same as that suggested 

 here for the Betulaceae, inasmuch as the near relations of both 

 genera are most numerous in warmer countries. 1 



Rhythmic change of form is well illustrated by the succes- 

 sional change in the dimensions of the leaves and internodes 

 of a shoot produced during one vegetative season. The main 

 axes of herbs and the long-shoots of trees representing one 

 season's growth in length in cold-temperate regions generally 

 show a gradual increase followed by a sudden or gradual 

 decrease in the dimensions of the successive leaves and inter- 

 nodes. Branches of herbs display, according to position of 

 insertion, either a similar rise and fall in length of internode, 

 or a steady fall, or a rhythm transitional between these two. 

 It might be thought that in cold-temperate plants the longer 

 internodes owe their greater length to the higher temperature 

 of summer compared with those of spring and autumn ; but 

 this suggestion in relation to both trees and herbs is disproved 

 by the fact that growth in length is often concluded before 



