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IX. On the Modifications of j^stivation observable in certain Plants, formerly 

 referred to the Genus Cinchona. By Mr. David Don, Libr. L.S. 



Read April 2nd, 1833. 



1 HE various forms of aestivation appear to depend, in a great measure, on 

 the relative position and development of the organs of reproduction ; for we 

 have remarked that the valvate kind occurs most frequently in such flowers 

 as have those organs much enlarged and projecting beyond the mouth of the 

 tube of the corolla, or where there exists any considerable inequality in length 

 between the stamina and pistillum. It is much more varied in monopetalous 

 than in polypetalous flowers ; for, with the exception of a portion of the Ruta- 

 cece, principally from New Holland and South America, the imbricate form is 

 found generally to prevail in the latter class. In the valvate form the pieces, 

 having their edges sloped in opposite directions, are nicely fitted together, 

 affording not only the most complete protection to the delicate organs within, 

 but also ample space for the development and subsequent impregnation of the 

 stigma. 



Among the monopetalous orders the form of aestivation is a character of 

 such high value as oftentimes to afford the only palpable distinction to the 

 limitation of families ; but the extensive order Rubiacece presents a striking 

 exception, examples of almost every modification of aestivation being afforded 

 by it. The Rubiacece appear to constitute a grand central point of union (of 

 which several may be remarked in the vegetable kingdom,) between the fami- 

 lies of the monopetalous class ; and possessing great diversity of form and 

 character, they are found to partake more or less of the habit and structure of 

 those orders to which they are related. The Rubiacece are intimately allied 

 on the one hand to Caprifoliacece and Falerianece, and on the other to ^ipo- 

 cyneoe and Gentianece, being distinguished from the two former by their sym- 

 metrical flowers, and from the latter by their adherent ovarium and undivided 



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