172 



MORPHOLOGICAL DEVELOPMENT. 



flowers of each umbellule there are decided differences: the 

 central flowers being small and radial, while the peripheral 

 ones are large and bilateral. But in other genera, where not 

 only the flowers of each nmbelhile but also the umbellules 

 themselves, are closely clustered into a flat surface, the umbel- 

 lules themselves become contrasted; and many remarkable 

 secondary modifications arise. In an umbel of Heracleum, 

 for instance, there are to be noted the facts; — first, that the 

 external umbellules are larger than the internal ones; 

 second, that in each umbellule the central flowers are less 

 developed than the peripheral ones; third, that this greater 

 development of the peripheral flowers is most marked in the 

 outer umbellules ; fourth, that it is most marked on the outer 

 sides of the outer umbellules; fifth, that while the interior 

 flowers of each umbellule are radial, the exterior ones are 

 bilateral; sixth, that this bilateralness is most marked in 

 the peripheral flowers of the peripheral umbellules; seventh, 

 that the flowers on the outer sides of these peripheral 

 umbellules are those in which the bilateralness reaches a 

 maximum; and eighth, that where the outer umbellules 



touch one another, the flow- 

 ers, being unsymmetrically 

 placed, are unsymmetrically 

 bilateral.* The like modifi- 

 cations are displayed, though 

 not in so clearly-traceable a 

 way, in an umbel of Tordy- 

 lium, Fig. 252. Considering 

 how obviously these various 

 forms are related to the vari- 

 ous conditions, we should be 

 scarcely able, even in the 



* I had intended here to insert a figure exhibiting these differences ; but 

 as the Cow-parsnip does not flower till July, and as I can find no drawing 

 of the umbel which adequately represents its details, I am obliged to take 

 another instance. 



