140 Transactio7is. 



admixture of vegetable humus — surface of tuffs composed for the most part 

 of pumice. The tuffs themselves are of an extremely porous character, and 

 any noticeable water-holding capacity the soil may possess is due to the 

 addition of humus. The nature of the soil, then, is such as to hinder the 

 growth of forest, which depends as a formation on an abundant supply o£ 

 atmospheric moisture. 



On the lower more level ground a layer of black loam 10-20 cm. deep 

 rests on pumice tuffs. Water percolates freely ; hence the ground is usually 

 more or less parched. On the hilltops, however, the more abundant supply 

 of moisture promotes a luxuriant growth, and the consequent decay of 

 vegetation adding humus to the soil increases its water-holding capacity. 

 No water lies on the surface, though the soil is usually moist,* and a layer 

 of dead leaves prevents it rapidly becoming dry. On the hillsides the 

 weathered surface of the tuffs forms the soil. 



The density of the forest-roof, by regulating the supply of rain and 

 light, has a direct bearing on the distribution of the undergrowth. The 

 vegetation on the forest-floor varies from a dense growth of ferns 2 m. high 

 to a few scattered ferns and seedhngs, whilst in places it is entirely absent, 

 there being only dead leaves and branches lying about. As a general rule, 

 where the foliage is most dense there is little undergrowth, but where a 

 break occurs ferns and other plants are abundant. On the hilltops, how- 

 ever, where clouds hang about in northerly weather, more moisture will be 

 supplied to the undergrowth, which is therefore usually most luxuriant, 

 notwithstanding the fact that the forest-roof may be not less dense than 

 it is on the lower ground. In Denham Bay there is everywhere a dense 

 undergrowth. Climatological Table II shows that on Sunday Island 

 three-fourths of the rainfall occurs during northerly weather. Rain-clouds 

 impinging against Big and Expedition Hills drop their moisture on the 

 summits of these hills and on the flat in Denham Bay directly billow. 



(2.) Trees : The trees are all evergreens. One only exhibits partial 

 defoliation. In the winter months Homolanthus poli/andrus has appreciably 

 fewer leaves on it than it has in the spring and summer. The older leaves 

 fall away in the autumn, leaving only those on the young shoots, and usually 

 less than 10 cm. across. 



Two sizes of trees are distinguishable in the dry forest — those which form 

 the upper or third tier of vegetation, which varies from 15 m. to 20 m. in 

 height, and those growing in the shade of these, and forming a second tier 

 of vegetation. Four trees are included in the first category — Metrosideros 

 villosa, Corynocarpus Icevigata, Myoporum Icetum, Melicytus ramiflorus. In 

 the wet forest a third tier of vegetation cannot be distinguished, the general 

 height of the formation being about 10 m. 



Over the greater portion of Sunday Island a moist climate and dry 

 soil require that the trees should be able to endure frequent short droughts, 

 and at the same time hold their own against hygrophytes. The soil decides 

 the battle in favour of the plants least affected by drought. Rolled leaves, 

 which I suspect are an adaptation to situations exhibiting alternately wet 

 and dry conditions, are a conspicuous feature in the Sunday Island vegeta- 

 tion, not on account of the number of species, but because of the preponder- 

 ance of individual plants, possessing them. In Rapanea kermadecensis, an 

 important plant in dry forest, and Coprosma petiolata, a characteristic 

 shrub of the coastal formations, the whole leaf rolls inwards so that opposite 

 sides of the upper surface may be touching. Both these plants are endemic ; 

 hence the adjustment of their organs to the environment is complete. 



