THE GYMNOSPERMS 



361 



of the pines, spruces, etc., has a peculiar structure. The pollen 

 grain has on either side two rounded air sacs formed by a swelling 

 out at these points of the outer layer of its wall. These serve as 

 floats, and the pollen grains are very buoyant, being carried great 

 distances by air currents. The mature pollen grain is a rudi- 

 mentary male 

 prothallium or 

 gamete plant 

 (gametophyte) . 

 While it is de- 

 veloping two 

 sterile prothal- 

 lial cells are 

 formed which 

 soon disappear 

 in the pines. 

 The rest of the 



Fig. 346. Fig. 347- 



Section of staminate Two sporo- pollen grain is 

 cone, showing sporan- phylls removed, 



gia. showing open- a rudimentary 



ing of spor- 

 angia, sperm case (an- 



theridium) and consists of two cells, the tube cell (or wall 

 cell) and the sperm case cell (antheridial cell), or central cell 

 (see fig. 348). 



529. The pistillate cone and ovules. The pistillate cone 

 (or carpellate cone) forms the fruit. These cones are also usually 

 developed in clusters. They are developed from a whorl of buds 

 at the end of the shoots. They begin their development in 

 spring, and the axis is soon covered with spirally 

 arranged scales. On the inner and upper face 

 of each scale, at its base, are two ovules which 

 correspond to large spore cases (macrospor- 

 angia). At the lower end are two horn-like 

 processes, and between these is an opening, 

 the micropyle. The wall of the ovule is called the integu- 

 ment. The interior tissue of the ovule is called the nucellus. 

 It is in truth the real tissue of the large spore case. A large 



Fig. 345- 



Stamina te cone of white 

 pine, with bud scales re- 

 moved on one side. 



Fig. 348. 



Pollen grain of 

 white pine. 



