76 MARGARET C. FERGUSON 



consists of three cells. The axial row represented in fig. 144, for 

 instance, is a beautiful object, clearly and definitely differentiated 

 from the surrounding tissue, 3^et there is not the least ground 

 for supposing that the upper cell has ever divided. Such a 

 figure as this represents the characteristic axial row in Pinus 

 Strobiis and P. rtgida, while the axial row of four cells illus- 

 trated in fig. 145 is typical for P. austriaca. This point has 

 not been sufficiently studied in the two other species to admit of 

 generalizations for them. The axial row, then, varies from 

 three to four cells in the same species, but there is a tendency 

 in some species to form three and in others to form four cells. 



Significance of the Tetrad DivisionWithin the Ovule. — We 

 have observed that at a certain point in the development of the 

 ovule in Pinus a centrally located cell becomes differentiated 

 from those surrounding it by its greater size and the more 

 vacuolate character of its cytoplasm. This cell after under- 

 going a period of growth and rest gives rise to the reduced 

 number of chromosomes by a peculiar method of division known 

 as the heterotypical division, and this mitosis, as is characteris- 

 tic in spore formation, is quickly followed by a second division, 

 at least in the lower cell. The basal cell resulting from this 

 last division passes through a season of growth extending over 

 several weeks, as we shall shortly see, and finally, by repeated 

 divisions, gives rise to the female gametophyte. The process 

 of division is in all essentials exactly similar to that which takes 

 place within the microspore-mother-cell, and results, as there, 

 in spore-formation. Nuclear phenomena attending the early 

 development of the female gametophyte have not been carefully 

 investigated until comparatively recent times, but wherever 

 studied the conclusion has been unhesitatingly drawn that in 

 the ovule, as within the anther, a true spore-formation takes 

 place. 



The essential character of a spore is, manifestly, not that it 

 should have a certain arrangement relative to its sisters within 

 the mother-wall, neither is the presence or absence of a wall of 

 vital importance to its existence unless, indeed, the spore is to 

 be disseminated. Rosenberg ('01) finds the pollen-grains to be 

 filiform in Zostcra and arranged side by side ; Strasburger ('01) 



