ON COMPOUND FLOWERS. 217 



Mrs. B. It is a very natural mistake. It is, indeed, 

 a sort of calyx common to the whole head ; but as each 

 floret has its separate calyx (and a calyx of so peculiar a 

 nature as not to be overlooked,) this common calyx is 

 distinguished by the name ofinvolucrum, analogous to the 

 involucrum which surrounds the receptacle of the umbel. 



Emily. There seems to be a considerable resemblance 

 between the umbel and the head of flowers ? 



Mrs. B. That is very true. If you conceived the 

 branches of an umbel to be so extremely short that they 

 could not be distinguished, the umbel would be similar 

 to a head ; and this is exactly the case of the Eryngiums. 



Emily. The involucrum of the China Aster differs, 

 however, in one respect from that of an umbel, the brae- 

 teas of which it is composed being much more numerous, 

 and disposed in several rings or whorls around the stem. 



Mrs. B. That is the case with the greater number of 

 compound flowers, but it is not universal ; for in the 

 Salsafy (Tragopogon,) the Orthonna, and several others, 

 the bracteas, or, as they are more commonly called, the 

 folioles of the invotucrum, are placed in a single ring. 

 When disposed in several rings, they are sometimes 

 equal ; at others, vary in si?e : they are sometimes curl- 

 ed up ; at others spread out. Some are soft, others 

 scaly ; and there are some which terminate in a species 

 of thorn or prickle, as in the thistle. These varieties in 

 the nature of the folioles serve to distinguish the numer- 

 ous class of heads of flowers, which constitute no less 

 than one-twelfth part of the vegetable kingdom. 



Caroline. It is then, indeed, very necessary to make 

 acquaintance with so numerous a body of plants. But 

 you have mentioned both compound flowers, and flowers 

 growing in a head : are these terms synonymous? 



Mrs. B. Certainly not; and I am obliged to you for 

 reminding me of a want of accuracy. 



All flowers which shoot in numbers from a common re- 

 ceptacle, either flat, or slightly elongated, are called heads. 



1187. Into what mistake does Caroline fall 1 ? 1188. Under what 

 circumstances would the umbel be similar to a head 1 ? 1189. In what 

 respect does the involucrum of the China Aster differ from that of an um- 

 bel'? 1190. How do the bracteas vary in different flowers'? 1191. 

 What portion of the vegetable kingdom may be thus distinguished'? 

 1192. What flowers are called heads'? 



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