170 PRINCIPLES OF BOTANY, ETC, 
Many other examples will be found by an attentive 
dissection of flowers. 
§ 155. 
From the singular structure of plants may be de- 
duced the aphorism, that those genera, which have 
this singular structure, are more easily ascertained 
than those that come near to the natural structure. 
This last extends over all the natural families of the 
vegetable kingdom. ‘The umbelliferous plants, the 
lilies, the papilionaceous flowers, the cruciform and 
compound flowers, are, on account of the similarity 
of their structure, with difficulty distinguished. For 
ascertaining with facility the genera of every kina, 
rules have been laid down which must be adapted 
to new discovered plants. ‘There are rules which in 
general are applicable to all plants, and others that 
regard only particular families. But before proceed. 
ing to these we shall endeavour more accurately to 
define the calyx. 
§ 156. 
In some flowers that have but one external cover, 
it is difficult to determine whether that cover is calyx 
or corolla. Various methods have been devised to 
ascertain this, but never with success. Indeed we 
‘do not apply to any purpose the difference between 
calyx and corolla; we can give them both the same 
name; we may call the calyx the outer, and the 
corolla the inner cover. ‘This would in uncertain 
cases remove any doubt of what was calyx and what 
3 | : corolla 5 
