702 MR JOHN MCLEAN THOMPSON ON 



Developmental details will be given later for all the floral parts of Greyia Suther- 

 landii, and these will be correlated. 



It is proposed to deal at length in subsequent publications with the evolution of 

 zygomorphy in various affinities, and what has been stated here is intended as an intro- 

 duction to developmental studies, in which correlation as affecting the development of 

 floral parts will be fully stated. 



The author wishes to express his indebtedness to the Carnegie Trust for their 

 assistance in the publication of the accompanying plates. 



Summary. 



When any member of a cycle of floral parts shows a definite tendency to reach a 

 developmental stage in advance of the remaining members, or to have the attainment 

 of a developmental stage delayed, zygomorphy is initiated. 



The zygomorphy may be only temporary, in which case morphological actinomorphy 

 may be restored. 



Zygomorphy is a progressive character in Greyia Sutherlandii. It appears first in 

 the perianth, and later in the androecium and gynoecium. 



The androecium is at first actinomorphic. but becomes antero-posteriorly zygomorphic. 

 Lateral zygomorphy then follows in Greyia. 



Actinomorphy is almost or entirely restored in the perianth, while lateral zygo- 

 morphy continues for a time to dominate the androecium ; but the latter becomes 

 morphologically actinomorphic when all the anthers have dehisced. 



The restoration of actinomorphy progresses from the perianth to the androecium. 



No anther dehisces until the stamen-filaraent reaches the maximum length. 



The attainment of the maximum length of stamen-filament immediately precedes 

 the dehiscence of the anther. 



The main details of the anther are completed while the androecium is actinomorphic 

 in the bud. 



The staminal zygomorphy is due to differences of filament-length, but it disappears 

 when they have all attained full maturity. 



In Staphylea and other actinomorphic flowers the anthers are completed at an early 

 stage in development. The full number of filament-cells is produced about the time of 

 completion of the anther. 



The great increase in stamen -length immediately previous to anther-dehiscence is 

 primarily due to the elongation of the filament-cells (extension). 



The examination of the stamen-filaments of Greyia Sutherlandii shows that the 

 antero-posterior zygomorphy in the young bud is caused by differences in the cambial 

 activity in the filaments, and the later lateral zygomorphy is mainly due to differences 

 in the rate of cell-elongation. 



