THE PLANT ECOLOGY OF THE COAST BELT OF NATAL. 879 



greater general cloudinesSj etc., is less than at higher alti- 

 tudes. The general effects of the greater chemical intensity 

 of sunlight with increasing altitude were described in a 

 former paper. ^ Various species with a wide range, e.g. 

 Helichrysum adenocarpum, Dierama pendula, 

 Mor^ea spathacea, show remarkable colour clumges in 

 their flowers with ascending altitude, passing from white on 

 the coast through pale pink to deep crimson or even purple 

 on the Drakensberg. 



Actinometer measurements are constantly being taken in 

 connection with the photographing of the vegetation types. 

 In the early part of the day the atmosphere is often remark- 

 ably clear, and the sensitive paper darkens to the normal tint 

 in about two seconds. It is the only satisfactory time, as a 

 rule, for good photographic work. About 10 a.m. the atmo- 

 sphere becomes more hazy and later the sky may become 

 overcast. 



Owing to the prevalent development of scrub and tree- 

 growth generally there is always plenty of shade, except 

 where the ground has been cleared. The lightest shade is 

 shown by the thorn trees (Acacia spp.), but even these 

 reduce the amount of light to as little as '02 of the normal at 

 midday when the sun is overhead, though the numerous 

 plants which grow up underneath the pioneer thorn trees get 

 full sunlight earlier or later in the day when the slanting- 

 rays penetrate underneath the umbrella-shaped canopy. In 

 dense scrub and forest, where numerous climbing plants 

 assist in closing in the canopy, the shade is often so dense 

 that only cryptogamic species of plants can grow underneath, 

 and regeneration on the floor of the forest ceases until gaps 

 are produced by the death or destruction of some of the 

 dominant trees. But even where Acanthaceous herbs, etc., 

 are found the light intensity may only be •002 of full 

 sunlie-ht. Amono- the shrubs the Rubiaceous Psychotria 

 cape n sis is one of the deepest shade resisters. I have 



' Bews, J. W., " The Plant Ecology of the Drakensberg Range."' 

 ' Annals of the Natal Museum; voL iii. pt. 3. 1917. 



