THE INITIATION OF STAMINAL ZYGOMORPHY. 701 



formed the top of each filament. The largest number of cells counted in a filament- 

 row was 121, and such numbers as 118 and 120 were found. These large numbers 

 were observed in stamens in which the lateral zygomorphy had just been initiated, and 

 120 was the largest number of cells counted in a longitudinal row of a fully grown 

 stamen whose anther had dehisced. 



In short, the antero-posterior zygomorphy in the young bud was caused by difierences 

 in the cambial activity in the filaments, and the later lateral zygomorphy was mainly 

 due to differences in the rate of cell-elongation. 



It will now be understood that the restoration of morphological actinomorphy was 

 accomplished in the expanded flowers by the elongation — sooner or later — of practically 

 the same number of cells in each filament. It will be evident that the lateral 

 zygomorphy was the outward expression of differences in the rate of elongation of the 

 filament-cells. It should be further stated that in the first-formed flowers of Greyia 

 Sutherlandii examined, while the lateral zygomorphy was quite pronounced — as will 

 be seen by the numbering of the stamens in fig. 24, — the entire anterior stamen-group 

 dehisced before any of the posterior stamens were fully elongated. 



The lateral zygomorphy was found to be much more pronounced in later-formed 

 flowers, and in fig. 25 — which is a floral diagram of such a flower — it will be noticed 

 that, of the first five stamens to dehisce their anthers, two belong to the posterior group. 



The zygomorphy was a progressive character. 



Distinct lateral zygomorphy may be observed in the androecium of many expanded 

 flowers, but in others — as in the Rhododendrons — it has been observed by the writer only 

 in the bud stages (fig. 13), and in those, when the flowers have expanded, it has usually 

 been replaced by a definite antero-posterior zygomorphy. 



It is to be noted that, while the antero-posterior and lateral zygomorphy dominated 

 the bud stages and young expanded flowers of Greyia Sutherlandii, the inequalities in 

 the floral parts, which were then so evident, were sooner or later counterbalanced. 



In certain Rhododendrons, however, while the lateral zygomorphy disappears as 

 development proceeds, the antero-posterior zygomorphy persists to the end. 



It is proposed to designate zygomorphy of the type above described for Greyia 

 Initial Zygomorphy, for no permanent morphological alteration was recorded for the 

 androecium, and the final condition of the perianth and style was but slightly zygomorphic. 



In marked contrast to this is the condition of the stamens in such a form as 

 Verhascum thapsus L., in which, in the expanded flower, not only is inequality in 

 stamen-length produced, but permanent modifications in stamen-form and in filament 

 and anther construction are also established (fig. 16). 



An attempt has here been made to compare briefly the external form of some parts 

 of flowers which possess the zygomorphic character, at various stages, with their 

 corresponding internal conditions as shown by cell-size. The result has been the 

 recognition of the part played by cell-elongation in the production of a zygomorphic 

 androecium. 



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