684 TEXTBOOK OF BOTANY 



cones in the terminal buds, sometimes as early as the autumn preceding 

 the spring in which the cones mature. Early in the spring the tetrad 

 of microspores is formed from the mother cell, and the number of 

 chromosomes becomes monoploid. At first the microspore is a cell with 

 a single nucleus. It germinates about a month before the pollen is shed, 

 and nuclear and cell division results in a generative cell and a tube 

 cell." The outer spore wall of the pollen grain has by this time enlarged 

 and become separated from the inner wall, forming two balloon-like 

 sacs. The cells within the pollen constitute the male gametophyte of the 

 pine. 



Pollination. At this stage the pollen sacs dehisce and the released 

 pollen grains are blown about by the wind. Pollen grains are produced 

 in prodigious numbers, and their yellow color gives the soil and objects 

 nearby the appearance of having been sprinkled with powdered sulfur. 

 A few of these pollen grains fall upon the ovulate cones in which the 

 cone scales are at this time somewhat separated, and some of them 

 come in contact with the megasporangia. 



The ovulate cone. The larger, or ovulate, cones develop singly or, 

 more often, two or three close together near the upper end of the new 

 stem segment. Each cone consists of an axis upon which the cone 

 scales, or "megasporophylls," are spirally arranged. The sporophylls of 

 the young cone are small green scales that enlarge and become woody 

 at maturity. The ovulate cones remain on the trees for nearly two years 

 and on the trees of some species indefinitely. 



Each megasporophyll has on the upper side of its scale-like portion 

 two megasporangia or ovules. The ovule consists of an oval body of 

 tissue (nucellus) enclosed by a single integument containing an open- 

 ing (micropyle) near the base of the sporophyll. 



In each ovule of the very young ovulate cone a megaspore mother 

 cell becomes difl^erentiated, and upon two successive divisions there re- 

 sult four megaspores arranged in a row. Reduction division occurs 

 when the spore mother cell divides. Each megaspore has the monoploid 

 number of chromosomes. The megaspore most distant from the micro- 

 pyle enlarges as the other three disintegrate, and later the female game- 

 tophyte develops from it within the ovule. 



Gametophytes and fertilization. Pollination occurs in May or June 



' The nucleus of the microspore first divides into two vegetative nuclei, one of which 

 degenerates. The other daughter nucleus then dixides, and one of the resulting nuclei 

 together with its cytoplasm becomes the antheridial cell; the other nucleus degenerates. 

 It is from this antheridial cell that the generative and tube cells are formed. 



