Dr Dickson on Diplostemonous Floivers^ etc. 241 



Agrostemma, Cerastium (e.g. C- triviale),'^ Lasiopetalum 

 (e.g. L. cori/UfoUum),^ Lilium, &o. (PI. III., fig. 2.) Com- 

 paratively few dicotyledons, but almost all diplostemonous 

 monocotyledons, fall under this bead. 



2dj That in which the younger stamens are the more ex- 

 ternal, and the carpels, when of the same number, alternate 

 with the older stamens. Examples — Geranium, Erica, 

 MalacJmim, &c. (fig. 1.) As I have already mentioned, the 

 greater number of diplostemonous dicotyledons fall under 

 this head. 



Tn the first of these two forms of diplostemony (that in 

 which the younger stamens are the more internal), the ar- 

 rangement requires no explanation, as it corresponds with 

 the ordinary centripetal evolution of successive whorls upon 

 an axis. 



The case, however, is widely different where the younger 

 stamens are the more external. Here we seem to have a 

 centrifugal succession of parts upon an axis. Have we any 

 analogies to guide us in explaining this apparent anomaly ? 

 What suggests itself most naturally in this regard is per- 

 haps the case of polyadelphous flowers, where the members 

 of each staminal group are usually developed in centrifugal 



the statement, for there the circle of the younger stamens appears almost 

 to coincide with that of the older ones, or, if anything, to be somewhat smaller 

 than it. In the more advanced stages, the younger stamens are yery distinctly 

 the more internal. At all events, the plant requires re-investigation upon this 

 point. 



* I have examined, with great care, the development of the stamens in 

 Agrostemma Flos-Jovis and Cerastium triviale, and in both of them I can confi- 

 dently state the younger stamens to be the more internal. I have given 

 figures of young flowers of these two plants (Plate III., figs. 12 and 13). Payer 

 has, on the contrary, stated that the older stamens in Cerastium are the more 

 internal ; but his figures do not indicate this very satisfactorily. To judge 

 from my own experience of C. triviale, I should imagine it to be very difficult 

 to determine which staminal whorl is the more internal, when the flower is 

 viewed so much from above as those represented by Payer (Organogenie, pi. 72, 

 figs. 7 and 8). In all flowers like Cerastium, where the receptacle is very con- 

 vex, it is very necessary to obtain a completely side view, so as to see the 

 ditfereuce in elevation of the different parts. In flowers with flat receptacles, on 

 the other hand, the view from above is the most advantageous, as the parts 

 are no longer on higher and lower elevations, but on larger and smaller circles. 



t Organogenie, pi. 9, fig, 4. — Though the fact is not mentioned in his text, 

 Payer's figure leaves no doubt as to the younger stamens (staminodes) in this 

 plant being on a smaller and more internal circle than the older. 



