BOTANICAL APHORISMS, -- 19} 
killed in our winters, and it must be restored by 
sowing it again. Other perennial plants with us are 
shrubs in warm countries, because no cold destroys 
their stems. When thus the duration ‘of a. plant 
exhibits any discriminating mark, the other species 
must be accurately examined to know whether they 
too are notot longer duration ina milder climate. But 
if plants vary in this respect in the same region, such 
must be considered as different species; for exam- 
ple, the Mercurialis annua and perennis resemble one 
another much, but he names express a distinct spe- 
cific difference. 
§ 187. 
Most plants are dsstinguished from one another 
their leaves. | 
Almost all plants are distinguished by the various 
form of their leaves. But there are instances where 
this character will not answer; for the umbelli- 
ferous, the compound, all the aquatic plants, figs, 
and mulberries are an exception. In these the 
leaves are subject to such considerable variations, 
that without much experience it is difficult to dis- 
tinguish a species from a variety. . When, there- 
fore, there is uncertainty in the leaves, other cha- 
racters must be resorted to. 
When plants differ from one another by their 
spines, stipulze or bracteze, they may be considered 
as distinct species. But it is to be observed, that 
these parts, if taken as specific characters, must not 
be subject to fall off. 
by 
4 
§ 188. 
