142 PltlNCItLES OP PLANT-TEEATOLOGY. 



When the corolla comes to resemble the calyx it 

 must represent a further differentiation away from 

 its original staminal structure, and hence this must be 

 held to be a progressive phenomenon. 



All these instances support the view that sepals and 

 petals are essentially the same organs and have had 

 an identical origin. 



Heteropetalodt. — Under this heading must be 

 ranged those cases in which, in flowers normally pro- 

 ducing more than one kind of petal, the one kind 

 changes into the other ; also those in which the petals 

 become changed into a form not normally found in the 

 genus. 



In certain varieties of marguerite {Ghri/santhemum 

 fniU'scens var.) and Pyrethrum the disk-florets of the 

 head become enlarged and coloured white like the ray- 

 florets, this representing a half-way stage towards 

 complete " doubling " of the head in which the further 

 character of zygomorphy is added to the disk-florets. 



In the snowball-tree [Viburnum Opulus var. sterilis) 

 all the interior flowers of the corymb have become 

 enlarged and white in colour like the peripheral ones, 

 which gives the striking, showy character to the 

 inflorescence of this variety. Now this feature, in 

 which all the flowers of the corymb are alike, is the 

 normal one in other species, e.g. the laurustinus (F. 

 Tinus) and V. macrocephalus, although in these cases, 

 of course, none of the flowers is sterile. 



The same phenomenon occurs in certain garden 

 varieties of Hydrangea, the interior flowers of the 

 corymb becoming as showy as the exterior ones. 



Labellum-pelory. — In orchids there are two kinds 

 of petals : the ordinary, lateral ones, and the posterior 

 median " labellum." The peculiar form of this latter 

 is held by some authorities to be due to the lateral 

 fusion with the original ordinary petal of the two 

 lateral stamens of the outer whorl. Rolfe describes a 

 flower of Gattleya labiata having an extra lip in place 

 of one of the lateral petals, this being supposedly due 



