PRINCIPLES OP NATURAL ARRANGEMENT. 343 



instead of beneath it (as in Fig. 129), it is called superior ; this 

 conformation is due to the adhesion of the calyx to the wall of 

 the ovarium, so that it forms a tube completely enclosing it, as 

 in the Rose, Apple, &c. In some plants the calyx is altogether 

 absent ; and then the general rule is that the corolla is likewise 

 deficient. Such plants are said to be Achlamydeous, the essential 

 parts of their flowers being destitute of envelope. In the Com- 

 posite, however, which possess a corolla, the calyx is present in 

 an undeveloped form, constituting the down or pappus, which 

 surrounds the bottom of the corolla, and is attached to the top 

 of the seed-vessel, as in the Dandelion. 



497. In regard to the number and regularity of the parts of 

 the corolla, nearly the same may be said as of the calyx. These 

 characters are valuable in some instances and not in others. The 

 separation or adhesion of the petals, constituting what is com- 

 monly known as the polypetalous or monopetalous corolla, is 

 often a character of first-rate importance, as will presently 

 appear. Still, from the cause formerly mentioned, (. 456) it is 

 liable to some uncertainty, and must not, therefore, be trusted 

 too implicitly. Sometimes no corolla is to be found, the calyx 

 still being present ; and the plant is then said to be apetalous 

 (destitute of petals), or Monochlamydeous (having but one enve- 

 lope). This is a character, however, on which great reliance 

 cannot be placed; since apetalous genera frequently present them- 

 selves, in orders which usually possess complete flowers, an 

 occurrence which is less common amongst monopetalous Exogens, 

 than among those which have separate petals, so that the 

 character is of more value in the former than in the latter. 



498. When our attention is directed to the more essential 

 parts of the flowering system, the stamens and pistil, we 

 meet with some characters on which more constant reliance can 

 be placed ; but these are not among the most obvious, such as 

 the inexperienced Botanist would first attend to. For example, 

 the number of stamens is a character to which little importance 

 can be attached ; since this is liable to vary extremely among 

 the genera of almost every order, in many cases among the 

 species of the same genus, not unfrequently, among individuals 



