418 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
Of the three parietal layers the outer becomes the endothecium 
and the inner the tapetum. The middle layer consists of tabular 
cells which, as development proceeds, become more and more 
compressed and finally disappear, probably being absorbed by 
the tapetum. The tapetal cells reach their greatest development 
at the time of tetrad formation, when they are large with heavily 
staining contents. Subsequently they undergo disorganization 
and disappear altogether during the growth of the microspores. 
Fig. 83. — 5, Cross section through the upper end of a mature flower, sep. 
sepals; pt, petals. 6, Longitudinal section of a younger flower showing a 
stamen of each of the two sets. 
The outer parietal layer in becoming the endothecium grows 
considerably in thickness and its cells develop on their radial 
walls riblike thickenings which unite into a plate on the inner 
wall. At the time of dehiscence the anther wall consists of only 
the endothecium and the epidermis. Dehiscence is by means of 
a longitudinal slit. 
