192 PRINCIPLES OF BOTANY, ETC. 
; § 188. 
The props, (fulcra), present certain specific cha- 
racters, which are to be preferred to all others. 
When plants differ from one another by their 
spines, stipule or bractese, 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. 
§ 189. 
The thorn, (spina), and the tendril, (cirrhus), are 
always to be taken as certain characters. 
The thorn is nothing more than an indurated im- 
perfect bud, which, when the plant grows in a lux- 
uriant soil, changes to a branch. Pears, oranges, 
and other plants in a poor soil produce thorns, 
which leave them in richer ground. Some plants 
that have many thorns, retain them even in fertile 
soils. ‘The prickle is very constant, and is never 
altered by change of soil. In the same manner the 
tendril changes in some plants with papilionaceous 
flowers. We must first be perfectly certain that 
the thorn or the tendril are never wanting before we 
distinguish the species by them. 
§ 190. 
The mode of inflorescence is a certain character. 
We have no instances of the mode of inflorescence 
being subject to variation. When plants differ m 
this respect they are undoubtedly different species. 
The number of the flowers, that is, whether they 
3 | be 
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