THE SHAPES OF FLOWERS. 165 



sided position; that its parts of attachment have rigidity 

 sufficient to prevent this attitude from being very mrvch in- 

 terfered with; and that though the individual flowers vary 

 somewhat in their attitudes, they do not vary to the extent of 

 neutralizing the differentiating conditions — there remains an 

 average divergence from a horizontal unfolding of the flower, 

 to account for its divergence from radial symmetry. 



We pass insensibly from forms like these, to forms having 

 bilateral symmetry strongly pronounced. Some such forms 

 occur among flowers that grow at the ends of upright stems; 

 as in Pinguicula, and in the Violet tribe. But this happens 

 only where, in successive generations, the flower unfolds its 

 parts sideways in constant relative positions. And in the 

 immense majority of flowers having well-marked two-sided 

 forms, the habitual exposure of the different parts to different 

 sets of forces, is effectually secured by the mode of placing. 

 As illustrations, I may name the genera — Orchis, Utricularia, 

 Salvia, Salix, Delphinium, Mentha, Teucrium, Ajuga, Ballota, 

 Galeopsis, Lamium, Stachys, Nepeta, Marrubium, Calamintha, 

 Melittis, Prunella, Scutellaria, Bartsia, Euphrasia, Rhinan- 

 thus, Melampyrum, Pedicularis, Linaria, Digitalis, Oro- 

 banche, Fumaria, &c; to which may be added all the Grasses 

 and all the Papilionacew. In most of these cases the flowers, 

 being sessile on the sides of upright stems, are kept in quite 

 fixed attitudes ; and in the other cases the peduncles are very 

 short, or else stiff enough to secure general uniformity in the 

 positions. A few of the more marked types are shown in 

 Figs. 234 to 241. 



S&s 



234 23S 2,30 237 ^S 2>3$ 240 241 



Very instructive evidences here meet us. Sometimes within 

 the limits of one genus we find radial flowers, bilateral 

 flowers, and flowers of intermediate characters. The genus 



