BETA VULGARIS X79 



is the only notable exception to the findings of Winge (xi) and 

 Heel (9). 



Since microsporogenesis proceeds at a more rapid rate than mega- 

 sporogenesis in any given flower, it is probable that the individual 

 flowers of a cluster are not ordinarily self-pollinated. The mature 

 pollen grain has a thick, sculptured outer wall with many thin 

 spots or germ pores, one of which serves as the point of emergence 

 of the pollen tube. The division of the microspore nucleus to 

 form the tube nucleus and the two microgametes occurs several 

 days before the liberation of the pollen. Prior to dehiscence, there 

 are also small irregular starch grains in the microspore which 

 disappear before the pollen is shed. There is a considerable amount 

 of pollen degeneration, but Artschwager and Starrett (3) find that, 

 in most cases, more than one pollen tube reaches the megagameto- 

 phyte. These grow down the stylar canal, and fertilization may 

 be eff"ected in lo hours or less. 



Megasporogenesis. — In megasporogenesis, the archesporium 

 is formed from a single hypodermal cell of the nucellus; and sub- 

 sequent periclinal divisions of epidermal cells result in the embed- 

 ding of the megaspore mother cell more deeply in the nucellar 

 tissue. The meiotic divisions produce a linear tetrad of mega- 

 spores, the outer three of which degenerate while the inner one is 

 functional. The orientation of the megagametophyte is at first 

 straight; but changes in the position and content of the nucellar 

 cells, together with their unequal growth rate, result in curvature; 

 and further divisions of the nucellar cells produce the tissue which 

 ultimately forms the perisperm of the mature seed. (Fig. iio.) 



The development of the megagametophyte follows the usual 

 program. There are three successive equational divisions of the 

 megaspore nucleus producing eight free nuclei. Of these, the egg, 

 or megagamete, and two synergids are located toward the micro- 

 pyle; the two polar nuclei unite to form the primary endosperm 

 nucleus, which is usually centrally located; and the three antipodal 

 cells are at the chalazal end. There may be more than three an- 

 tipodals which persist for several days after fertilization and then 

 disintegrate. 



Embryogeny. — At the time of fertilization, the ovule is small; 

 but later it grows rapidly and attains mature size before the embryo 

 within is fully developed. During this period^of growth, the calyx 

 also enlarges and its lobes curve back to the position held at anthe- 



