THE FLOWER AND FRUIT loi 



micropyle and chalaza are at right angles to the hilum. It differs 

 from the campylotropous type in that the nucellus is straight in 

 this type rather than curved. (Fig. 38, C) 



Occasionally, in the development of the ovule there is a non- 

 divergence of its parts. This results in conjoint growth of the 

 nucellus and integuments, or conjoint growth of the funiculus and 

 the integument. Frequently, in anatropous ovules, this leads to 

 the development of a ridge-like structure, the raphe. 



Megasporogenesis. — The nucellus begins its development as a 

 more or less localized activity of the hypodermal layers of the 

 sporophyll; and, as in microsporogenesis, the archesporial cells are 

 hypodermal. A hypodermal cell (or several of them) constitutes 

 the arches-porium, becoming larger than the adjacent cells. The 

 cells of the epidermal layer sometimes divide periclinally so that 

 several layers may develop from it, forming an epidermal cap. The 

 archesporium, which in most instances consists of a single cell, 

 may divide transversely, the outer daughter cell becoming the 

 primary parietal cell, and the inner one functioning as the primary 

 sporogenous cell. In other cases, the archesporial cell may function 

 directly as a megaspore mother cell. If a primary parietal cell is cut 

 off, it may undergo several periclinal divisions producing wall cells, 

 in addition to such as may have come from the epidermis. There 

 are many variations in the behavior of the primary parietal tissue, 

 from types in which there is a pronounced development to those in 

 which the parietal cells undergo few or no periclinal divisions. 

 Where there is no initial transverse division of the archesporial 

 cell, there is no development of parietal wall tissue. 



The behavior of the megaspore mother cell is markedly uniform 

 in the angiosperms, despite the fact that several types are recorded. 

 A linear tetrad of megaspores is formed, accompanied by reduction 

 division. Of the megaspores, the chalazal one of the linear series 

 is usually functional; and, as a result of successive nuclear divi- 

 sions, an eight-celled megagametophyte is formed. At maturity, 

 it consists of a megagamete and two synergids, located at the micro- 

 pylar end; three antipodals at the chalazal end; and two polar 

 nuclei which are more or less centrally located. Commonly, the 

 polar nuclei unite just prior to or coincident with their union with 

 one of the microgametes to form the endosperm nucleus. This occurs 

 in the process of double fertilization at the time when the mega- 

 gamete and microgamete unite to form the zygote. 



