THE EMBRYO SAC OF PHYSOSTEGIA' 
LESTER W. SHARP 
(WITH PLATES VI AND VII) 
The material of Physostegia virginiana (L.) Benth., upon which 
the present work is based, was collected near Alma, Michigan, in 
August 1909. Although the investigation has brought out no new 
point of fundamental importance, the results are deemed worthy of 
record. 
The ovule arises from the floor of the sporangial chamber as a 
small protuberance, which in growing pushes out the ovary wall 
in such a manner that it becomes completely surrounded by the 
latter except at the funiculus. At the time when the archesporium 
is distinguishable as a single hypodermal cell, the young ovule is 
slightly curved, and as growth proceeds this curving becomes more 
pronounced, until finally an anatropous condition is reached. 
A single massive integument is developed. 
The archesporial cell, which cuts off no parietals, grows rapidly, 
and is markedly elongated at the time when its nucleus goes into 
synapsis preceding the first division (fig. 1). This cell, which, on 
account of the occurrence of the heterotypic prophases in its 
nucleus, is to be regarded as the megaspore mother cell, by two 
successive divisions gives rise to a row of four megaspores (fig. 2). 
Of these the outer three degenerate (fig. 3), while the innermost 
enlarges and gives rise to the embryo sac. 
The nucleus of the functioning megaspore divides, and the two 
daughter nuclei take up positions near opposite ends of the sac, 
which becomes strongly curved, and, owing to rapid growth, 
develops a large central vacuole (fig. 4). Each nucleus divides, 
forming the four-nucleate stage (fig. 5). These four nuclei by one 
further division give rise to eight, and walls soon form, resulting 
in their organization into a typical egg apparatus, three antipodal 
* Contribution from the Botanical Laboratory of the Johns Hopkins University, 
No. 20. 
Botanical Gazette, vol. 52] [218 
