44 MR, W, В. B. OLIVER ON THE 
rich in guano and vegetable-mould, and on the more level places some leaf- 
mould has collected, but along the upper portions of the scrub the soil is the 
ordinary voleanie tuff devoid of humus. 
The scrub varies in height from three or four metres at its eastern end to 
six or eight near the west end of the island. 16 is composed of shrubs or 
small trees of irregular habit, but growing close together (see Plate 3), 
and bearing the dense dark green foliage at the top. The floor is thickly 
strewn with dead leaves and branches, and gives one the impression that 
deeomposition is slow. Dead branches and twigs attached to the living 
plants area conspieuous feature, and dead trees still standing are occasionally 
met with. It is evident that the plants have a hard struggle for existence, 
and possibly the fumes in the atmosphere act as preservatives, inhibiting the 
decomposition of the dead parts. Except where broken into by colonies of 
Gannets, the scrub ends on all sides abruptly and compactly, the foliage 
descending to the ground. Open scrub occupies а belt above the closed 
formation, near which the plants are low, rounded, detached bushes one metre 
tall, but. they gradually decrease in size and in number towards the inner 
edge of vegetation a little distance up the slope of the volcano. 
The scrub is a pure association of Metrosideros tomentosa, А. Rich., no 
other species being seen except near the coast, where patches of the broad- 
leaved tussocks, one metre tall, of Phormium tenax, Forst., may occur, while 
on the sea-cliffs a few shrubs of Coprosma Baueri, Endl., are found. 
Metrosideros tomentosa has the branchlets and under-surfaces of the leaves 
covered with woolly tomentum. The leaves are coriaceous with recurved 
margins, both surfaces with thick cuticle, and epidermis of a single layer of 
small cells. The mesophyll consists of three layers : (1) an upper, which may 
be water-storage tissue, of three or four rows of irregular thick-walled cells 
with few chloroplasts ; (2) then follow three rows of palisade-cells ; (3) below 
which is the spongy parenchyma of small cells. — Coprosma Baueri has 
shining dark green leaves with recurved margins. The cells of the upper are 
larger than those of the lower epidermis, and both have thickened outer walls. 
The mesophyll consists of three layers: (1) а single layer of large flat cells 
next the upper epidermis ; (2) three rows of palisade-cells of which the upper 
are very much larger than the others ; (3) spongy parenchyma containing air- 
cavities. Metrosideros robusta has likewise a mesophyll consisting of three 
or four rows of palisade-cells between a layer of large cells next the upper 
epidermis and the spongy parenchyma of close small cells. The leaf- 
structure of the shrubs of White Island is thus essentially the same, and is 
distinctly xerophytic in character. No stomata were detected on the upper 
surface of the leaves of any of the species. 
Perhaps the most nearly related formation in the New Zealand area is the 
scrub on Rangitoto Island in Auckland Harbour. Here is a lava-cone on 
which, over most of the surface, blocks of lava are irregularly piled, and rain- 
