648 Journal of Agricultural Research voi. xn, no. » 



partially or entirely the original mother cytoplasm (PL B, 9; PL C, 



I, 3, 4)- 



Pollen mother cell wall. — The history of the pollen mother cell 

 wall is of interest as it differs somewhat from that generally reported for 

 the higher plants. 



In the lily (Allen, i), grape (Dorsey, 12), and in many other plants in 

 which pollen development has been studied, it is usual, during prepara- 

 tion for the first meiotic division, for the pollen mother cells to separate 

 from one another, due apparently to dissolution of the middle lamella 

 and to growth of the anther walls, forming a greater space into which 

 the cells can round up and float free from one another. Allen (/, p. 200) 

 considers that the separation is due to a dissolution of the cell walls 

 from between the mother cells, and that each is "surrounded only by a 

 plasma membrane." Following separation, a very decided thickening 

 of the material surrounding the cytoplasm takes place (Pi. B, 9; 

 PI. C, i). This, Allen (j), Tischler {41), Stevens {38), and others speak 

 of as a thickening of the mother cell wall. Following the formation of 

 the tetrads, this material increases and, as the cells of the tetrad separate 

 from one another, flows between them. This material is usually of a 

 rather firm nature and in buckwheat {38) often persists for some time 

 after the liberation of the microspores from it. 



There is evidence in Fragaria spp. which indicates that this material 

 is entirely distinct from the mother cell wall and is in no way dependent 

 on it for its increase in volume, thus appearing to be more of the nature 

 of the gelatinous sheath which surrounds groups of cells in many of the 

 algae. 



In Fragaria spp., in place of the pollen mother cells rounding up just 

 before or during reduction division, while there is taking place a rapid 

 growth of the anther walls and a consequent increase in size of the 

 anther cavities, the cytoplasm separates at the angles from the walls 

 and rounds up independently (Pi. B, 9, 14, 15; Pi. C, t). The walls 

 remain in contact with one another and adjust themselves to the 

 increasing space by stretching. As soon as evidence of rounding up of 

 the cytoplasm appears, a gelatin-like material is secreted unevenly 

 about the cytoplasmic mass (PI. B, 9; Pi. C, i). This material is 

 apparently identical with that laid down about the plasmic mass in 

 the grape, lily, and forms like them in which the mother cell wall rounds 

 up, supposedly following the dissolution of the mother cell wall. In 

 these cases it is generally spoken of as the thickening of a new mother 

 cell wall. In the strawberry this material increases in amount until at 

 the completion of tetrad formation and before liberation of the micro- 

 spores, the spores are completely embedded in it (PL C, 3, 4). The 

 mother cell walls are still present, but simply divide the anther cavity 

 into large spaces, which are only partially filled by the tetrad (PL C, 

 3). An examination of analogous stages to these in the lily and 



