THE STAMIXATE CONE 369 



is a sort of inner bark and must be included in anv account 



t/ 



of this region of the stem. 



The functions of these tissues are discussed in Part I, 

 Chapter vm. 



352. The cones of the pine. The cones of the pine, as in all 

 conifers, are of two sorts: (1) staminate, when made up of stamens 

 (microsporophylls), and (2) carpellate, when composed of carpels 

 (megasporophy 11s) . 



The staminate cone. The staminate cones are developed in 

 clusters on the young growth that appears late in the spring 

 with the opening of the terminal buds (Fig. 298, A). Each 

 cone consists of a large number of stamens closely packed 

 together and arranged somewhat spirally around the central axis 

 (Fig. 298, B). The stamen bears two pollen sacs (Fig. 298, C, D), 

 within which the pollen grains are developed. The pollen grains 

 are formed in groups of four, or tetrads (Fig. 298, E), just like 

 the spores of the bryophytes and pteridophytes, and their further 

 history shows them to correspond exactly to the microspores. 1 

 The pollen sac is then a microsporangium, and the stamen a 

 microsporophyll. The pollen sacs develop from a group or region 

 of cells as in the horsetails, lycopods, and Selaginella, and not 

 from a single surface cell as in the common ferns. 



The pollen grains are produced in enormous quantities, and 

 being set free by the splitting of the pollen sacs, they are scat- 

 tered as fine yellow dust by the wind. Sometimes pollen is carried 



V v i 



from pine forests by the wind for many miles, falling as so-called 

 showers of sulphur. The pollen grains are especially adapted 

 for distribution by the wind, for the outer layer of the cell wall 

 is swollen on two sides to form outstanding wings (Fig. 298, F). 



1 This relationship is further established by the count of the chromosomes 

 in the Scotch pine (Pinus sylvestris), which shows that the pollen grain has 

 12, while certain tissues of the pine sporophyte have 24. Pollen formation 

 is then the period of chromosome reduction when the spojjpphyte generation 

 passes over to the gametophyte, as explained in Sees. 334 and 335. Similar 

 chromosome reduction undoubtedly takes place with the formation of the 

 embryo sac in the nucellus. 



