COHESION AND SUPPRESSION. 



251 



to form a " posterior " petal, its compound nature 

 being shown by the conspicuous apical notch. An 

 interesting example was seen in Dendrobiam sp. ; an 

 incompletely 2-merous flower had two sepals, fore and 

 aft ; and two petals ; of these there was one normal 

 lateral one, and another in the position of the labellum, 

 but its insertion extended round to that of the other 

 lateral petal, so that it can be easily pulled round into 

 that situation without any strain ; it had also in part 

 the colour of a labellum and in part that of a lateral 

 petal; hence it clearly represents a fusion between 

 the two. 



Fig. 151. — Fraxinus dipetala. Diagram of normal flower with two 

 petals (p). 



It is probable that in many other instances of 

 a 2-merous corolla fusion between two petals has 

 occurred. The same is true of many 4-merous corollas : 

 in the pansy in which a 4-merous calyx had arisen 

 owing to fusion of the two anterior sepals, the corolla 

 was also 4-merous, and it is probable, although no 

 direct evidence for it was seen, that in this whorl also 

 fusion between two members had occurred. 



Velenovsky describes flowers of Forsythia viridis- 

 sima in which the four petals were fused in pairs to 

 form a 2-merous corolla of two lateral petals ; the 

 calyx had the form of four green leaves (PI. LI, fig. 11). 

 It is interesting to note, as he points out, that the genus 

 Fraxinus (ash) has a species (dipetala) in which, as the 

 name denotes, the presence of two petals only is 

 normal (fig. 151). Velenovsky observed in a flower 

 of the rock-cress (Arabis alpina) that the petals had 



