THE EMBRYO SAC OF EPIPACTIS: 
WILtiam H. Brown AND LESTER W. SHARP 
(WITH PLATE x) 
The present study is based upon material of Epipactis pubescens 
(Willd.) A. A. Eaton, collected at Cold Spring Harbor, N.Y., in 
August 1909. 
The archesporium is distinguishable very early as a single 
hypodermal cell which terminates an axial row surrounded by a 
single epidermal layer. As growth proceeds, the young ovule 
becomes strongly anatropous and develops two integuments, the 
outer one being continuous with the slender stalk. Since the nor- 
mal heterotypic prophases always occur in the nucleus of the . 
archesporial cell preparatory to its first division, it is to be regarded 
as the megaspore mother cell, no parietals being formed. 
The subsequent course of development to the complete embryo 
sac is not identical in all ovules, the same end being reached by a 
variety of methods. The behavior in what probably represent the 
Majority of cases is as follows. After becoming considerably en- 
larged, the megaspore mother cell undergoes its first division (fig. 1). 
Since the spindle lies near the micropylar end of the mother cell, 
the resulting daughter cells are very unequal in size (fig. 2). The 
larger, chalazal cell again divides unequally, forming two mega- 
Spores, while in only a single case was the micropylar daughter 
cell observed in process of division (figs. 3 and 4). The innermost 
megaspore enlarges and gives rise to the embryo sac, while the 
other cells of the row soon degenerate. 
The nucleus of the functioning megaspore divides to two (fig. 
5), and very soon small vacuoles appear in the cytoplasm, mostly 
in the region between the two nuclei. Meanwhile the sac grows 
considerably, but continues to have the general shape of the mega- 
spore mother cell. As growth proceeds, the increase in volume of 
the cytoplasm fails to keep pace with that of the sac cavity, so that 
Contribution from the Botanical Laboratory of the Johns Hopkins University 
ro Po a t of th bry written by Mr. SHa 
, 
oe. oy 7 
and the discussion by Mr. BRowNn. 
430] [Botanical Gazette, vol. 52 
