36 MARGARET C. FERGUSON 



not been previously described. A broad open space, repeatedly 

 figured between the daughter-spores and the mother-wall, has 

 been invariably attributed to shrinkage ; but it is probable that, 

 in some cases at least, it represents this thickened wall which 

 has failed to be differentiated with the stains used. Wiegand ('99) 

 says that the spores of Potamogeton are as if imbedded in a 

 ground mass of some viscid substance, but he does not figure it 

 and makes no statement regarding the development of cell-walls 

 between the microspores. 



Origin of the Air-sacs. — As soon as the young microspores 

 have become enclosed, each within its own special chamber of 

 the mother-cell, it is evident that a special wall has been de- 

 veloped about each spore. This is doubtless secreted by its 

 own cytoplasm and is not, as Juranyi thought, derived from the 

 inner wall of the microspore-m other-cell. The spore-wall while 

 still very delicate becomes differentiated into an inner and an 

 outer layer corresponding to the intine and extine of the pollen- 

 grain. The young microspores are characterized by the rela- 

 tively large size of their nuclei, the nucleus filling almost the 

 entire cell just prior to the discharge of the spores. The cyto- 

 plasm which fills the remainder of the cell is in the form of a 

 loose reticulum (figs. 46, 47). 



As time goes on the outer wall of the microspore expands at 

 two points on opposite sides of the spore. A resistance is met 

 with in the thick wall of the spore-mother-cell and the plastic 

 inner wall of the microspore responding to this new pressure 

 becomes indented along the surfaces corresponding to the ex- 

 tended portions of the outer spore-wall. Thus a clear open space 

 having in section the form of a biconvex lens is formed between 

 the extine and the intine on either side of the microspore. 

 These are the beginnings of the wings or air-sacs that are so 

 conspicuous in the mature pollen-grain of the AbietinccB. 

 Finally the pressure becomes so great that the mother-wall is 

 ruptured and the spores are liberated (figs. 47, 48). Coulter 

 and Chamberlain ('01) noted the fact that the wings make their 

 appearance in Pinus Laricio while the microspores are still 

 within the mother-cell, but they recorded no observations regard- 

 ing the origin and development of these sacs. Strasburger and 



