PLANTS IN CITIES. 85 
and pouring forth oxygen gas. And it is very 
certain they do pour out a Jittle carbonic-acid 
gas at night, but it is very small in quantity, 
and seems to arise simply by evaporating from 
the leaves, since the sap of plants always contains 
a little of this gas dissolved in it. 
Wherever plants are best exposed to sun- 
light, as a general rule, there they are the 
strongest and healthiest. Every one who lives 
in a great smoky city, and takes pleasure in 
trying to rear a few plants on his narrow window- 
sill, must have wondered and at the same time 
lamented over their pale and sickly look, as 
compared with the deep green hues of the 
country plants of the same species. The rea- 
son is simply this, that the thick state of the 
air, and the tall houses around, so diminish the 
force of the light, that the poor plants only 
receive a tithe of that received by those which 
luxuriate on some sunny bank, looking to 
the warm south, in the country. On this 
account, trees which grow alone, and are there- 
fore exposed on all sides to the light, are well 
known to form more tough and durable tim- 
ber, than such as grow in thick forests where 
