68o 



A TEXTBOOK OF THEORETICAL BOTANY 



towards the base of the cone scale (Fig. 68 1). At the inner end, which 

 corresponds to the apex of the nucellus, a small uncovered aperture is left, 

 called the micropyle. The integument is fused to the nucellus except for a 

 short distance near the micropyle. Near the apex in the nucellus is a single 

 large archesporial cell. It divides to form a single tapetal cell and a 

 megaspore mother cell. The nucleus of the megaspore mother cell divides 

 meiotically into four thus giving rise to a tetrad of four megaspores. Of 

 these only one develops into a large cell which is termed, from analogy with 

 the Angiosperms, the embryo sac. It is important to recognize that in 

 Pimis only one megaspore mother cell is formed and that only one of its 

 megaspores continues to develop. At the time of pollination only the 



Integument 



Micropyle 



Nucellar 

 beak 



Archegonia 



Prothallus 



Ovuliferous 

 scale 



Fig. 68 1. — Pimis sylvestris. Longitudinal radial section of an 

 ovuliferous scale passing through one of the ovules. 



megaspore has been formed and its development into the embryo sac is all 

 subsequent to that event. 



Pollination 



The transference of the pollen grains to the ovule is effected by the wind. 

 Towards the end of May the microspores are liberated in vast quantities, of 

 which some small part falls on the scales of the female cone. The edges of 

 the bract scales become inrolled, thus forcing the ovuliferous scales apart, 

 which allows the pollen to gain access to the ovules. Prior to this the scales 

 have been tightly closed around the axis of the cone, enclosing and protecting 

 the rudiments of the reproductive organs, and after pollination they close 

 again, remaining so until the seeds are ripe. 



In addition to the opening of the scales the nucellus secretes a drop of a 

 mucilaginous fluid in which the microspores become entangled. After 

 pollination this drop of liquid dries up, drawing the microspores through 

 the micropyle to the apex of the nucellus, where they are held by the dried 

 mucilage in a position suitable for germination. This position is ensured 

 by the air sacs which float the microspore in the pollination drop with the 



