THE STAMEN 137 



development may be rapid just before the flower opens — largely as the 

 result of cell elongation— and may continue after the flower is open. 



Ontogeny of the Sporogenous Regions. Study of the development of 

 the microsporangium in angiosperms began about the middle of the 

 nineteenth century; it was a popular field of morphological study from 

 about 1870 to 1900, when many taxa were investigated. In the large 

 amount of detailed information available, much confusion in interpreta- 

 tion and terminology exists. This applies particularly to the description 

 of the anther wall and its ontogeny, where topographical terms have 

 been applied loosely. This is not surprising. Most of the early studies 

 were purely descriptive and were rarely compared with one another. 

 Variation in structure of the anther wall (which has been generally 

 assumed to be the sporangium wall) is great. Many of the descriptions 

 were based on the theory that the stamen was cauline in nature, and 

 special attention was given to origin of specific layers from "derma- 

 togen" or "periblem." Deeply sunken sporangia were unknown; the 

 sporangium was considered superficial. Anther wall and sporangium 

 wall were often considered synonymous terms; commonly, the wall of 

 the pollen sac was called the sporangium wall. The possibility that 

 sterile tissues of the sporophyll may form part of the sporangium wall 

 did not enter the picture. The loss of layers in the wall during ontogeny 

 — even of the epidermis — was usually overlooked. Only with recogni- 

 tion of sporangium position as primitively sunken in the sterfle tissues 

 of the sporophyll has there been a basis for interpretation of the mor- 

 phology of the anther wall. 



Part of the difficulty of interpreting the nature of the several wall 

 layers outside of the sporogenous tissues lies in the variation in number 

 of these layers. The stamen has, unfortunately, been commonly looked 

 upon as a simple organ, readily interpreted morphologically; actually, 

 it is the most highly specialized of floral organs, resembling an ancestral 

 laminar sporophyll far less than the carpel, and showing much greater 

 variety in form. Adaptation to various types of pollen distribution has 

 brought about not only great range in external form but in the his- 

 tological structure of the anther. The sporangium, originally buried in 

 sterile tissue, seems to have become "superficial" and, in the highest 

 types, is largely free from the surrounding sterile tissues. Variety of 

 wall structure, seen in the many intermediate stages in this series, with 

 phylogenetic and sometimes ontogenetic loss of cell layers, obscures 

 the story of modification. 



Early in the ontogeny of a typical anther, when the beginning of 

 anther form is evident in the primordium, groups of cells in the center 

 of each of the four corners can be distinguished from the cells around 



