422 SECTIONAL TRANSACTIONS.— K. 



a dense oak-hazel woodland, whereas in the latter Aira ccespitosa dominates, 

 and brambles and saplings are very gradually becoming established . 



Response to competition is also affected by the ability of such plants as 

 Capsella bursa-pastoris to mature seed rapidly at various seasons of the year. 



Miss A. C. Halket. — The periodic movements of the flowers of some nyct- 

 anthous plants (3.5). 



The results of a study of the flowers of some nyctanthous plants are 

 presented. The flowers studied were Melandryum album, Melandryum 

 noctiflorum, various species of Silene (S. Vallesia, S. nutans, S. ciliata, 

 S. tartarica, S. Zawadski, S. maritima), Schizopetalon Walkeri and Matthiola 

 bicornis. 



The flowers of different species open for a varying number of evenings, 

 many of them opening about six times. The hours of opening and closing 

 vary with the weather ; in most species the buds expand later in the evening 

 than the older flowers. On wet days the flowers remain open. 



The ' closing ' of the flowers is due to the inrolling of their petals. The 

 petals close gradually as they lose water and open as they absorb it. The 

 percentage of water is greater in open than in closed petals. 



Comparison of the areas of petals when closed and open shows that they 

 change considerably in size. The changes in area are due to the contraction 

 or expansion of the cells as water is lost or gained. 



The petals increase in size as they grow older ; their movements, how- 

 ever, are not growth movements but depend on their water-content, and 

 are related to the anatomical structure of the petals and the nature of their 

 cell-walls. 



Dr. G. Taylor. — The British Museum expedition to the mountains of East 

 Africa (3.30), 



The expedition visited four groups of mountains in East Africa and was 

 organised to make a comparative study of the flora and insect fauna at higher 

 altitudes, to obtain specimens and to ascertain whether any comparable 

 peculiarities existed between the plants and insects. It will be some years 

 before the collections are fully worked out, and at present it is possible only 

 to describe the characteristic vegetation of these regions and the elements 

 represented in this unique flora. 



Perhaps the most striking feature is the altitudinal zonation of the vegeta- 

 tion and the manner in which it tends to repeat itself on the different 

 mountain groups. In the lowest zone the plants are characteristic of the 

 African plains, but the type of vegetation changes through tropical forest 

 and dense bamboo forest until one meets on the one hand plants strongly 

 reminiscent of Europe and, on the other hand, grotesque forms which are 

 characteristic only of these African mountains. The vegetational zones may 

 be quite sharply defined, but, as a rule, they overlap for a considerable depth 

 and their altitudinal range may vary on the different mountains and even on 

 each side of the same mountain. Occasionally, but very rarely, the bamboo 

 zone may be absent. 



It is in the highest zone that specialisation of the species has proceeded 

 furthest, and the same genus is usually represented by different species on each 

 mountain top and frequently, but particularly in the arborescent Senecios, 

 these species are local endemics. Each mountain can be regarded as an 

 island arising from a sea of tropical vegetation, each island having its own 

 characteristic plants, though all are of the same general type. 



