THE MEGASPORANGIUM 73 



Schnarf, 1929) there is a multicellular archesporium and also an 

 extensive parietal tissue. In some members of the Rosaceae the 

 cells of the nucellar epidermis also divide periclinally and thus add 

 to the wall tissue (see also page 60). 



Megasporogenesis. The megaspore mother cell undergoes the 

 usual meiotic divisions to form a tetrad of four cells. The first 

 division is always transverse and gives rise to two dyad cells (Fig. 

 4QF-G). Typically the second division is also transverse and re- 

 sults in a linear tetrad of four megaspores (Fig. 52 A). Frequently 

 the micropylar dyad cell divides in a plane at right angles to that 

 of the chalazal dyad cell. This results in a T-shaped tetrad in 

 which the two outer megaspores lie in contact with the third mega- 

 spore which separates them both from the chalazal megaspore (Fig. 

 52B). Since both linear and T-shaped tetrads may occur in ovules 

 of one and the same ovary, it is unnecessary to give specific ex- 

 amples. Tetrads of an intermediate type (Fig. 4QH), in which the 

 wall separating the two micropylar megaspores lies at an angle of 

 approximately 45° with respect to the chalazal megaspores, are also 

 not infrequent. 



Rarely, the two upper megaspores of a tetrad lie in a line parallel 

 to the long axis of the ovule but the lower two lie at right angles 

 to it. Such 1-shaped tetrads are sometimes found in the Onagra- 

 ceae and have been reported in Zauschneria (Johansen, 1931a), 

 Anogra (Johansen, 19316), and Ludwigia (Maheshwari and Gupta, 

 1934). 8 Among other examples of a similar kind may be cited 

 Drimiopsis (Baranow, 1926), Tacca (Paetow, 1931), Styrax (Cope- 



8 The occurrence of 1-shaped tetrads in the Onagraceae is probably related to 

 the fact that here the micropylar megaspore gives rise to the embryo sac, while 

 the three chalazal megaspores are nonfunctional (see Chap. 4). 



prophase. (After Dahlgren, 1927.) C, Chrysanthemum corymbosum, nucellus, show- 

 ing megaspore mother cells each with four megaspore nuclei; one cell at the bottom 

 has lagged behind. (After Dahlgren, 1927.) D, Solanum melongena, ovule show- 

 ing hypodermal megaspore mother cell and two other such cells in the tissues of 

 the integument. (After Bhaduri, 1932.) E, Jussieua repens, nucellus, showing 

 multicellular archesporium. (After Khan, 1942.) F, Oncidium praetextum, ovule, 

 showing supernumerary archesporial cell arising from the nucellar epidermis. 

 (After Afzelius, 1916.) G, Limnanthes douglasii, normal archesporial cells in 

 nucellus and supernumerary archesporial cell in integument. (After Fagerlind, 

 1939.) H, Machaerocarpus californicus, megaspore mother cell in prophase. (Af- 

 ter Maheshwari and Singh, 1943.) 



