CAUSES INTERFERING WITH 
attributing a portion of the depressing effects 
upon some plants to deficiency of light ; but that 
this cannot be the sole cause is clear from the 
impossibility of growing such plants as ferns 
and mosses, which can, in any part of London , 
obtain as much light as they require. 
Want of sufficient moisture , again, cannot be 
the sole cause , as long before I began to grow 
plants in closed cases, my plants in the open air, 
although constantly supplied with moisture, all 
perished; and if we examine old walls in Lon- 
don, which, from some cause, are constantly wet, 
although we find vegetation, that vegetation is 
not healthy. The conditions for mosses may be 
so far fulfilled as to allow of the growth of their 
leaves ; and we shall everywhere see, on such 
walls, the silvery tips, when not obscured by soot, 
of Bryum argenteum ; but we must go two or 
three miles out of London before we find it in 
fructification. We may, it is true, occasionally 
find the Funaria hygrometrica ,* but this is an 
exception to the general rule. 
* The Funaria hygrometrica is a remarkable moss, differing widely 
in its powers of adaptation, and consequently in its greater geo- 
graphical range, from most of its congeners. It appears to delight as 
much in heat, as other mosses in cold. There is nothing in its 
structure to lead us to infer such a difference of constitution. Most 
mosses are confined within narrow limits, and will not fructify but 
under certain conditions. The Funaria is found in fruit not only in 
