176 PRINCIPLES OF BOTANY, ETC. 
§ 164. 
The external appearance, (habitus), of all thé species 
of a genus, must likewise be dttended to, but no generic. 
characters taken from it. 
This rule is to be taken with many restrictions, 
test by too rigid an adherence to it the science may 
be mjured. In new genera we must take care 
that the habit does not agree with that of other ge- 
nera; for it often happens that a plant, supposed to 
belong toa new genus, belongs to one already known, 
and varies only in the number or figure of the parts 
of the flower. 
When a plant agrees’ in flower and fruit with 
those of a genus already established, but is of a 
very different habit, it must not on that aecount be 
separated. An example will illustrate this: sup= 
pose a person to discover a plant, which in flower 
and fruit was a perfect Tilia, but had’ an herbaceous 
stem and pinnated leaves: however much the habit 
might differ from that of the other species of Tilia, 
the plant onght to be referred to that genus. This 
example is not really found in’nature, but similar 
ones are frequent. To exemplify the rule I shall 
however take a real instance from the same genus. 
There is a tree in North America whose fruit agrees 
with that of our Tilia, but in the flower there ap- 
pear, besides the petals, smiall petal-like scales; the 
habit, however perfectly agrees with that of the lime- 
tree; and as the flower differs only in that incon- 
siderable circumstance, the plant. is properly re- 
ferred:to the genus Tilia. 
§ 1656 

