1885.] 
Plants and the Structure of their Organs. 
219 
VIII. the relations between the 
STATION OF PLANTS AND THE STRUCTURE 
OF THEIR ORGANS. 
HIS subject has been recently studied by Professor 
\ T olkens, who has published a summary of his results 
in the “ Annals of the Royal Botanical Gardens at 
Berlin ” and in the “ Naturforscher.” He has especially 
investigated the action exerted by dry stations and climates 
upon the structure of the vegetative organs. Three methods 
for the examination of these relations are possible, and they 
have all been employed by Volkens. Either we may com- 
pare specimens of one and the same species, but from 
different stations, or we compare all the species of an order 
(or the orders of a family), or, lastly, all the typical plants 
of certain dry regions are submitted to a comparative 
examination. 
In opposition to Sorauer’s hypothesis, that evaporation is 
not a physiological but a mechanical process — an interpreta- 
tion which offers no basis for the explanation of the pro- 
tective agencies for checking transpiration — Volkens 
maintains that we have here to do with a purely physical 
process depending essentially on two factors, — the vapour- 
tension of the ambient air, and the nature of the evaporating 
membrane or liquid. As watery vapour is given off, both 
from the entire surface of the epidermis and through the 
interstices from the membranes lining the intercellular 
spaces, the evaporation will be the greater the more the 
surface and the intercellular system are developed. 
The evaporation is greater from the epidermis and from 
the internal membranes, because the vapour-tension is 
generally smaller outside of the plant than within it. Under 
certain circumstances the epidermis requires an especial pro- 
tection, in order to lessen the evaporation. This is effected 
both by thickening and cuticlisation of the membranes, and 
by a sliminess of the cellular juices. 
The adaptation to station appears very distinct in Poly- 
gonum amphihium, two varieties of which, terrestre and 
natans, are often formed from each other. Though the stem in 
the aquatic form is considerably thicker than in the land form, 
the stem-tissue of the latter is twice as thick as that of the 
