Mar. II, 191S Sterility in the Strawberry 649 



grape shows the mother cell wall lying closely about the gelatinous 

 material in which the spores are embedded, it having not disappeared 

 at the period of rounding up of the pollen mother cells, as is generally 

 assumed. At the time of rounding up of the pollen mother cells in these 

 two plants, there is no evidence of old walls being left behind, but there 

 seems rather to be a separation of the contiguous walls, due to the dis- 

 solution of the middle lamella and rounding up of them with the plasma 

 masses. The walls give a slightly different staining reaction from the 

 thick secreted mass and so can be readily distinguished from it. During 

 the liberation of the spores in the grape, it is not uncommon to see this 

 gelatinous sheath completely disappear, leaving portions of the original 

 mother cell wall about the spores. This gradually dissolves, liberating 

 the spores. In the strawberry, the procedure is much the same. The 

 gelatinous sheath, which shows no wall closely about it but only a limit- 

 ing membrane, dissolves, liberating the spores into the large cell bounded 

 by the mother cell wall. This soon disappears. 



At the time of liberation the spores have a distinct wall about them- 

 selves, which is independent of the surrounding sheath. Apparently 

 the condition in the lily, grape, and strawberry is identical, as far as 

 structures are concerned, but differs primarily in the separation of the 

 walls of contiguous cells in the former plants, while in the latter only 

 the cytoplasm of the pollen mother cells rounds up, leaving the walls 

 and middle lamella intact. 



Following the liberation of the tetrads and the disappearance of the 

 mother cell walls, degeneration of the tapetum takes place. Tapetal 

 degeneration seems to be in some way correlated with the disappearance 

 of the middle lamella from between the mother cell walls, for in those 

 forms which show an early rounding up of the mother cell walls there 

 is a correspondingly early degeneration of the tapetal cells. 



Growth of the; microspores. — Following the liberation of the 

 microspores, there is a period of very rapid growth in their walls. At 

 first this causes the microspores to become very irregular in outline 

 (PI. C, 7), but as growth continues and the wall becomes thicker the 

 cells become more spherical. The growth of the microspore wall is 

 so rapid that there is not a corresponding growth of the cytoplasm, 

 a condition which results in a large vacuole occupying the greater portion 

 of the cell cavity. When gr«wth of the microspores is nearly complete, 

 the original cytoplasmic content of the spore is spread out over the 

 periphery of the cell and about the nucleus (PI. C, 8). The compara- 

 tive size and consequently the very great decrease in relative cytoplasmic 

 content, of the newly liberated microspores and those ready for the 

 microspore division may be realized by comparing figures 5 and 8 of 

 Plate C, which are drawn to the same scale. 



Early in the development of the liberated microspore the wall is 

 differentiated into the extine and intine (PI. C, 7). The extine gradually 



