150 MORPHOLOGICAL DEVELOPMENT. 



sufficient to prevent tliis attitude from being very mucli 

 interfered with. ; and that though the individual flowers vary 

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

 neutralizing the difierentiating conditions — there remains an 

 average divergence from a horizontal unfolding of the flower, 

 to account for its divergence from radial fejrmmetry. 



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 Pinguicida, and in the Yiolet 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 that have well-marked two-sided 

 forms, the habitual exposure of the difierent parts to different 

 sets of forces, is efiectuallj^ secured by the mode of placing. 

 As illustrations I may name the genera — Orchis^ Utricular ia^ 

 Salvia, Salix, Delphinmny Mentha, Teucriiim, Ajuga, Ballota, 

 Galeopsis, Lamium, Stachys, GlecJioma, MarruJ)iiim, Cala- 

 mintha, Clinopodium, Melittis, Prunella, Scutellaria, Bart- 

 sia, Euphrasia, Rhinanthus, Melampyrum, Pedicularis, Lm- 

 aria, Digitalis, Orohanche, Fumaria, 8^g. ; to which may be 

 added, all the Grasses and all the PapilionacecB, 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 stifl* enough to 

 secure general uniformity in the positions. A few of the 

 more marked tj^es are shown in Figs. 234 to 241. 



Yery instructive evidences here meet us. Sometimes with 

 in the Kmits of one genus we find radial flowers, bilateral 

 flowers, and flowers of intermediate characters. The genus 

 Begonia may be instanced. In B. rigicla the flowers, various 



