1884.] Constitution of Gaseous Heavenly Bodies. 537 
Assures forms a more suitable substratum than a smooth 
one. he most easily contented plants in this respedt are 
the Bromeliacese, which Aourish even on the smoothest 
baiks, and thus serve as pioneers for other plants. Their 
loot-bodies are often quite covered with other Epiphytes. 
Many Epiphytes are limited to some particular trees ; for 
example, Trichomanes sinuosum to tree-ferns. Aspidium 
sesquipedale and A. nodosum occur in Trinidad only on palm- 
trees at the base of whose leaf-stems they And a sufficiently 
rich and moist substratum of humus, which might not be 
easily found elsewhere in the relatively dry climate of this 
island. In other countries they are less restricted in their 
station. 
If the foliage of a tree is too dense the development of 
epiphytic vegetation is prevented, as light and rain do not 
And sufficient admission. Thus the mango tree, whose dark 
crown is shunned even by birds, is almost entirely free from 
Epiphytes. 
The disappearance of this class of plants on the seashore 
is probably due to the salt present in the atmosphere. 
Notwithstanding numerous cases of exclusive adaptation, 
the general resemblance of the epiphytic Aora to that which 
in tropical regions covers rocks and stony ground cannot be 
overlooked. Bromeliacese, CaCtacese and ferns are very com- 
mon both on trees and rocks, though the species are not 
identical. But the rock Aora is the poorer on account of the 
less favourable substratum, in which deep Assures are 
wanting, the absence of dew, and the less advantageous 
illumination, which is either diredt sunlight or deep shade. 
VI. ON THE CONSTITUTION OF GASEOUS 
HEAVENLY BODIES. 
ROF. A. RITTER has been engaged with certain 
investigations relating to the period of development 
of the Axed stars, the colour of the twin stars, the 
entire duration of the visible existence of the Axed stars, 
and the present condition of Sirius. He remarks, in the 
“ Naturforscher,” that the whole duration of the visible 
existence of a Axed star is divided into three periods of 
VOL. VI. (THIRD SERIES) 2 N 
