THE M1CR0SP0RANGIUM 



31 



the mother-cell stage was found in September, and fully formed 

 pollen in the spring while the ground was still frozen. Dug- 

 has also observed that the microsporangia of Symplo- 



gar 



carpus pass the winter in the mother-cell stage. The pollen 

 mother-cells of Podophyllum peltatum are forming the tetrads 

 when the young plant has reached the surface of the ground, 

 so that in all probability the winter is passed in the mother-cell 

 staae. Although Conrad 29 found stamens well formed in the 

 winter buds of Quercus velutina, 

 the tissue of the anther was still 

 homogeneous. These records mere- 

 ly serve as an indication of what 

 may be expected when the subject 

 is really investigated. It is evident 

 that the time elapsing between the 

 differentiation of the archesporium 

 and pollination is often much longer 

 than has been ordinarily supposed. 

 It would seem probable that in gen- 

 eral those plants whose flowers open 

 early in the season, as most trees 

 and the vernal herbs, develop their 

 microsporangia before the end of 

 the " growing season," and that the 

 mother-cell stage is the usual win- 



Fio. 8. — A, Populus monilifera, 

 probably spore mother-cell stage, 

 Jan. 25, 1895 ; x 600. B, Cory lug 

 americana, pollen ready for shed- 

 ding, Dec. 7, 1897 ; x 400.— After 

 Chamberlain. 30 



ter condition. In the case of such 

 plants, therefore, the earliest stages 

 in the history of the microsporangia 

 must be looked for during the latter 

 half of the growing season that pre- 

 cedes the season of " blooming." 

 This suggests that the natural end of a growing season for the 

 sporophyte is the attainment of the mother-cell stage by its spo- 

 rangia, which is really the limit of the sporophyte in the alterna- 

 tion of generations ; and the natural beginning of the next season 

 is the reduction division and the beginning of the gametophyte. 

 Of course such a distinction disappears in many plants whose 

 seasonal habits are different from those we have been consider- 

 ing, but it suggests a natural division of growth between seasons, 

 and even in annuals the mother-cell stage is a prolonged one. 



