THE INITIATION OF STAMINAL ZYGOMORPHY. 695 



posterior stamen had been equalled by that of four other stamens. Those latter had, 

 however, outstripped the posterior stamen, and had differentiated into two pairs, of 

 which the more anterior were the longer (fig. 3). 



On the other hand, the greatest elongation had taken place in the anterior stamen, 

 and the four stamens which, in the preceding stage, had equalled it in length, were 

 differentiated into two pairs, of which the more posterior were the shorter (fig. 5). 



The stamens were then divisible into two groups of five — the one group anterior, 

 the other posterior — and, when this zy gomorphy had been completed, the shortest 

 stamens of the anterior group possessed filaments of greater length than the longest 

 members of the posterior stamens. 



This condition of the androecium was, however, not long maintained, for a lateral 

 zy gomorphy was initiated (fig. 8). 



It should be mentioned at this point that the flowers were borne in racemes, and 

 that, passing from the lower and first-formed flowers to those higher and later-formed, 

 a marked increase in the degree of this lateral zygomorphy was traced both in the bud 

 and in the expanded flower. 



During the development of the antero-posterior zygomorphy, the maximum rate of 

 stamen elongation was reached by the anterior stamen ; but, with the initiation of 

 the lateral zygomorphy, the most rapid elongation was no longer performed in this 

 stamen, but in those immediately to the right side of it, the flower being viewed 

 adaxially. In the lower or first-formed flowers the maximum rate of elongation was 

 maintained until dehiscence by the stamen immediately to the right of the anterior one ; 

 in the higher and later-formed flowers it was attained by the stamen second on the right. 



Transitional conditions existed. 



The immediate result of this new zygomorphy was the production of an asymmetric 

 androecium (fig. 8). Previous to its initiation the stamens had possessed one plane of 

 symmetry, and for each of the four stamens on the left of this plane a stamen was 

 found on the right of the plane of symmetry, in a similar position, and equalling it in 

 length (figs. 3, 4, and 5). In the later asymmetric condition, however, elongation had 

 become more rapid in the four stamens which had stood on the right of the plane of 

 symmetry — the flower being viewed from the posterior — and this increase in rate of 

 growth reached a maximum in one of the two stamens immediately to the right of the 

 anterior one (figs. 8, 9, and 10). With the introduction of the laterality, the equality 

 of length by which the stamens had been paired no longer existed, and thus, in 

 each expanding bud examined, the lateral zygomorphy was developed to a greater 

 or less extent. 



In late-formed buds — borne high on the raceme — the new zygomorphy made its 

 appearance at an earlier stage, and became more pronounced than in earlier-formed 

 flowers. 



The condition of the androecium — shortly before the opening of the bud — in certain 

 of the flowers borne high on the raceme is represented in fig. 15. An examination 



