Dr Dickson on Diplostemonous Flowers, (&c. 247 



the more internal (as in Agrostemma, &c.), the carpels, when 

 of the same number, alternate with the younger stamens ; 

 but that, where the younger whorl of stamens is the more 

 external (as in Geranium, &c.), the carpels alternate with 

 the older stamens.* 



The position of the carpels in the first of these two forms 

 requires no explanation, since it is manifestly in accordance 

 with the usual rule of alternation of floral whorls. 



In the second form of diplostemonous arrangement (that 

 in Geranium, &c.), the case is apparently very different. 

 Here the carpels alternate with the older stamens, and are 

 thus superposed to the stamens developed next to them 

 in order of time. If the outer and younger stamens in this 

 form be regarded as forming a true staminal whorl, and 

 as of equal value with the older whorl, we must admit a 

 very extensive series of exceptions to the rule of alternation 

 of whorls. On the other hand, if we view the younger 

 stamens here as forming a merely adventitious whorl, the 

 symmetry becomes at once intelligible, since the stamens 

 with which the carpels alternate are then the only ones of 

 primary importance. The fact of my interpretation of this 

 staminal arrangement satisfactorily explaining away such a 

 large number of apparent exceptions to the rule of alterna- 

 tion of whorls, is, I think, no small argument in favour of 

 its being well founded. 



It is further to be noted that, when, in a group of plants 

 exhibiting this pseudo-diplostemonous arrangement, the 

 outer and younger stamens disappear, the position of the 



* Some may be inclined to think that the circumstance of the carpels oc- 

 cupying different positions in these two cases is not a point of much import- 

 ance; that the carpels are only growing out where they have most space for 

 expansion. It is quite true, as matter of fact, that the carpels here do occupy 

 the places where they have most room ; but it appears to me impossible to re- 

 flect at all upon the arrangement of the parts of flowers, and admit that this 

 arrangement is primarily dependent upon any such simple law of packing, if 

 I may so express it. That such a law cannot be viewed as the basis of the 

 arrangement of floral parts, must, I think, be apparent, when the not un- 

 frequent instances of superposition of successive whorls are considered ; for 

 in these instances the parts are certainly not developed where there is most 

 room. The fundamental conditions are more likely to be found in the modi- 

 fications of a contracted spiral, than in the mere influence of surrounding- 

 parts upon nascent structures. 



