242 THE FLOWER. 



exterior in the bud, and therefore inserted lower on the axis than 

 the rest, the third is intermediate, and two others are entirely 

 interior, or inserted higher than the rest. In fact, they exactly 

 correspond with a cycle of the quincuncial, or five-ranked, spiral 

 arrangement, projected on an extremely abbreviated axis, or on a 

 horizontal plane, as is at once seen by comparison with Fig. 172, 

 173. Also when the parts are in fours, two are almost always ex- 

 terior in the bud, and two interior. Moreover, whenever the floral 

 envelopes, or the stamens or pistils, are more numerous, so as to 

 occupy several rows, the spiral disposition is the more manifest. 

 It is most natural, accordingly, to assume that the calyx, corolla, 

 stamens, &c., of a pentamerous flower are each a 

 depressed spiral or cycle of the f mode of phyllo- 

 taxis (239), and those of the trimerous flower are 

 similar spirals of the mode (238). But then the 

 parts of the successive cycles should be superposed, 

 or placed directly before each other on the de- 

 pressed axis (Fig. 171) ; whereas, on the contrary, 

 they almost always alternate with each other in the flower, as 

 in the annexed diagram (Fig. 279). 



442. To reconcile this alternation with the laws of phyllotaxis 

 in alternate leaves, Prof. Adrien de Jussieu has advanced an in- 

 genious hypothesis. He assumes the T 5 ^- spiral arrangement (241) 

 as the basis of the floral structure both of the trimerous and penta- 

 merous flower, (at least when the envelopes are imbricated in the 

 bud,) this being the one that brings the successive parts most 

 nearly into alternation, either in threes or in fives ; as will readily 

 be observed on inspection of the tabular projection of that mode, 

 given on page 147. The difference between the position of parts 

 in regular alternation, whether in threes or fives, and that assigned 

 by an accurate spiral projection of the -j- 5 ^ mode, is very slight as 

 respects most of the organs, and in none does the deviation exceed 

 one thirteenth of the circumference ; a quantity which becomes 

 nearly insignificant on an axis so small as that of most flowers, es- 

 pecially towards its narrowed apex. Moreover, if the interior or- 

 gans of a regular and symmetrical flower were thus to originate in 

 the bud nearly in alternation with those that precede them, they 

 would almost necessarily be pushed a little, as they develope, into 



FIG. 279. Cross-section of the flower- bud of the trimerous Tillaea, Fig. 277, to show the al- 

 ternation of parts. 



