BOTANICAL SOCIETY OF LONDON. 55 
of the earth, where they are subjected to a much greater 
pressure; — thus the diminution in the size of plants is gene- 
rally in a nearly uniform ratio with our ascent from the mean 
level of the globe, and vice versa. 
In a climate like England where the mean of Barometric 
pressure is, on an average of years, 29,500 inches, but little 
calculations are made for the effects of pressure, although on 
many occasions plants are exposed to a greater or lesser 
degree than at others. Many Botanists, however, consider 
that as heat is the prime mover of vegetable organization, by 
keeping the Thermometric column at the same height for 
Myrtles, as marked on the graduated scale, is all that is 
necessary ; when, were they but to observe the effects upon 
the foliage when the Barometer is at twenty-nine or thirty 
inches, they would see the necessity of an increase of tempe- 
rature when the Barometer is very low and vice versa ; un- 
less an easterly or north-easterly wind prevail. From this it 
will be seen how to preserve plants reared between the tro- 
pics (where the Barometer is nearly at the same height;) 
for these plants are much affected by the fluctuations of the 
mercury in the tube of the Barometer. 
In conclusion, I feel persuaded were Botanists to investi- 
gate strictly the laws of atmospherical phenomena, and com- 
bine in their researches Meteorology with the science of 
Botany, their field of observation would become greatly en- 
larged, and their plants preserved. 
FEBRUARY 16 th. 
J. E. Gray, Esq., F.R.S., President, in the Chair. 
Mr. W. H. White read a Paper, being a translation of M. 
Martens “ On Hybridity occurring in Ferns”* 
Ferns are generally regarded by Botanists as Agamous 
plants, which cannot be reproduced by seed. The sporules 
with which the undersides of their leaves are studded, are 
generally considered analagous to the little bulbs, which are 
often met with upon several Phanerogamous plants. Viewed 
in this light, Ferns could not produce hybrids ; or rather, 
* From the Bulletin of the Royal Academy of Brussels, No. 2, 1837. 
