Lecture XXIII. 



201 



It is contained in a thick cell-wall. The outer layer of this wall is 

 formed of cutin and bulging away from the inner layer forms two 

 large bladder-like outgrowths on the wall of the rounded mass. 

 These bladders are each about half the volume of the rounded 

 mass which lies between them. They contain air, and by decreas- 

 ing the specific gravity of the pollen-grain and greatly increasing 

 its surface they cause it to be easily carried on the wind. The 

 central mass is composed of one large rounded cell into which pro- 

 jects a hemispherical structure composed of a small disc-shaped 

 cell, the stalk-cell, and a small plano-convex cell, the body-cell. 



FIG. 64. Pinus silvestris, pollen-grain, x 250. That above, just abouMo 

 germinate, shows two transitory cells against the inside of the coat of 

 the pollen-grain. The lower grain has developed the pollen-tube. The 

 two dark bodies in the tube are the nuclei of the sperm-cells, the two 

 lighter are the nuclei of the tube and the stalk-cell respectively. 



Acquaintance with the previous types suggests that the pollen - 

 grains are spores and that the sacs which contain them are sporangia, 

 and if this is so, their relations to the stamens would indicate that 

 these latter are spore-bearing leaves sporophylls. A study of 

 the development confirms these surmises. The stamens arise as 

 excrescences on the side of the apex of the axis of a developing 

 pollen-cone. They correspond with leaves in their development 

 and in their relations to other parts. The pollen-sac arises on 

 these leaves as a mass of cells. The cells forming some of the 

 deeper layers of this mass differentiate as a group of thin-walled 

 cells completely filled with protoplasm and having large nuclei. 



