REPORT ON THE PIIILOSOPIIV OF BOTANY. 51 



Henslow has satisfactorily proved*, in part by the aid of a 

 monstrosity in the common Mignonette, and in part by a severe 

 appHcation of morphological rules, that my hypothesis must 

 necessarily be false ; and I am glad to have this opportunity of 

 expressing my full concurrence in his opinion. 



It has long been known that the ligulate and tubular corollas 

 of CompositcB are anatomically almost identical, and that their 

 difterence consists only in the five petals of the tubular corolla 

 all separating regularly for a short distance from their apex, 

 while the five petals of the ligulate corolla adhere up to their 

 very points, except on the side next the axis of inflorescence, 

 where two of them are altogether distinct except at their base. 

 M. Leopold von Buch explains this circumstance in the follow- 

 ing manner. He states that these ligulate corollas when vmex- 

 panded bear at their point a little, white, and very viscid body 

 or gland, which is a peculiar secretion that dries up when it 

 comes in contact with the atmosphere. The adhesion of this 

 gland is too powerful to be overcome by the force of the style 

 and stamens pressing against it from within. The corollas, which 

 are gradually curved outwards by the growth of those in the 

 centre of the inflorescence, at the same time bend down the 

 style, which consequently presses up against the line of union 

 of the two petals nearest the axis : although the style cannot 

 overcome the adhesion of the viscid gland at the point of the 

 corolla, it is able in time to destroy the union of the two inte- 

 rior petals, which finally give way and allow the stamens and 

 style to escape. As soon as this takes place, the corolla can no 

 longer remain erect, but falls back toAvards the circumference 

 of the capitulum, and thus contributes to the radiating character 

 of this sort of inflorescence. When the viscid body is either 

 not at all, or very imperfectly produced at the point of the co- 

 rolla, as sometimes happens in the genus Hieracium, especially 

 H. bifurcum, tubular corollas are produced instead of ligulate 

 ones. 



The ovulum is the organ where the greatest difficulty has 

 occurred in reducing the structure to anything analogous to 

 that of other parts. It is true that Du Petit Thouars regarded 

 it as analogous to a leaf bud ; but his view appears to have been 

 purely hypothetical, for I am not aware that he had any distinct 

 evidence of the fact. Some years ago M. Turpin, in showing 

 the great similarity that exists between the convolute bracteas 

 of certain Marcgraviacece and the exterior envelope of the 

 ovulum, took the first step towards proving that the hypothesis 



* Transactions of the Philosophical Society of Cambridge, vol, v. Part I. 



E 2 



