iQigl EVANS— PEN TSTEMON 429 



tion. It appears as a single enlarged and darker staining sub- 

 epidermal cell, which functions directly. Growth is quite rapid and 

 the cell soon becomes elongated. It is surrounded by a single 

 layered nucellus. The single integument forms along the sides 

 of the nucellus and soon surrounds it. A short time before the 

 integument has completely surrounded the ovule the megaspore 

 mother-cell has entered synapsis (fig. i). Miss Mitchell has 

 estimated that about 10 per cent of the ovules of Striga lutea have 

 reached synapsis at the same time. This percentage may safely 

 be placed much higher for P. secundifforus, probably more than 

 75 per cent of the ovules of a single ovary showing the same stage 

 of development. 



By the time the nucleus of the megaspore mother-cell has entered 

 synapsis the young ovule is rapidly assuming its anatropous form, 

 which is reached by the time the reduction divisions are completed. 

 The first reduction division occurs about the time the integument 

 has surrounded the ovule completely. This division is soon fol- 

 lowed by the second, forming the row of 4 megaspores. Either the 

 third or the fourth megaspore of the row may function in forming 

 the embryo sac (figs. 2, 3). The other 3 disintegrate rapidly and 

 become crushed by the growth of the one functioning. 



The megaspore which functions increases rapidly in size, the 

 micropylar end becoming bulbous while the chalazal end remains 

 narrowed (fig. 4). The chalazal end, however, lengthens rapidly 

 until it is 2-4 times as long as the bulbous portion. This growth 

 carries it to a point in contact with the end of the vascular system. 

 The nucellus early disappears, but by the time the embryo sac is 

 formed another nutritive layer, the tapetum, has formed from the 

 integument. During the growth of the embryo sac the single 

 nucleus by 3 divisions has formed the 8-nucleate sac. The rapid 

 growth of the sac causes the protoplasm to be much vacuolated. 



In the earliest stage of the 8-nucleate sac 4 nuclei are found 

 grouped at each end (fig. 4). Soon, however, a nucleus from each 

 end migrates toward the opposite end. Eventually they meet and 

 form the polar fusion nucleus (fig. 5). Balicka-Iwanowska 

 and ScHMiD have commented upon the place of this fusion. The 

 former says that it occurs near the middle of the sac, while the 



