THE OVULE 285 



microsporophyll makes this interpretation unsatisfactory. Similarity be- 

 tween anther and ovule has long been remarked; "anther sac and 

 nucellus ai-e homologous"; the parietal tissue of the anther is like that 

 of the ovule; "the ovule is more than a sporangium, as the anther is 

 more than four sporangia." In the anther, the sporogenous tissue is en- 

 closed by sterile tissues of the sporophyll; there is no evidence of a 

 sporangium wall as such. If so important a structure as a sporangium 

 wall is absent in the microsporophyll, the same condition would be 

 expected in the megasporophyll; homology of basic structure should 

 exist in the sporophylls. In both types of sporophylls, the sporogenous 

 tissue arises hypodermally and is buried below varying amounts of 

 sterile tissue — commonly called parietal cells in the anther and some- 

 times so called in the ovule. In the anther, this tissue seems to be a 

 remnant of the mesophyll of the sporophyll. If it is of similar nature in 

 the carpel — part of the tissues of an emergence — it is represented by 

 the nucellus. Close similarity exists in details of development and struc- 

 ture of anther-sac wall and nucellus. Specialization in both anther and 

 carpel is the progressive reduction of the surrounding sterile tissues 

 until only one cell layer encloses the sporogenous tissues externally. In 

 position and function, anther-sac wall and nucellus are alike, but 

 histology shows no distinction of a sporangium wall. If this wall were 

 present in ancestral angiosperms, some evidence of it might be ex- 

 pected in delimitation of sporogenous tissue in such primitive ovules as 

 those of Cosuarina and Cah/canthus, but this delimitation is not present, 

 although there are massive nucelli and many spore mother cells. 



The freeing of the fertile-tissue-bearing parts and their change to a 

 position more favorable for pollination — clearly seen in the stamen in 

 the bringing of the microsporangia from a sunken position, midway on 

 the sporophyll, to an exposed position, distal on the sporophyll — are 

 paralleled in the carpel, by the raising of the fertile tissue, sunken 

 among sterile cells of the lamina, to form an emergence consisting of 

 the fertile tissue still enclosed by sterile laminar tissues. Protection of 

 the sporogenous cells is given by surrounding cells (the nucellus) and 

 by adjacent laminar tissues including parts of the vascular meshwork 

 of the lamina (the integuments). In the more primitive families, the 

 ovular emergence is complex, including massive nucellus and two in- 

 teguments; in more advanced families, the nucellus is reduced to a very 

 few cells, even a uniseriate layer, and the integuments to one. A 

 parallel specialization is present in the highest types of anther, where 

 the anther-sac wall is reduced to a uniseriate cell layer and the connec- 

 tive is greatly reduced. 



The discarding of the fairly simple and convenient interpretation of 

 the nucellus as the megasporangium may seem to make the teaching of 



