THE PINE 155 



2. Open a pollen sac and mount the pollen grains (micro- 

 spores) in water. Under h.p. note (a) the two wings at- 

 tached to the pollen grain ; these are developed from the 

 outer wall of the cell ; (5) within the pollen grain the tube 

 nucleus lying near the center and the generative cell against 

 the wall at the side farthest away from the wings, also 

 occasional remains of a prothallial cell between the gener- 

 ative cell and the wall. Draw a pollen grain showing this 

 structure. 

 The pollen grains are developed in groups of four, tetrads, 

 within pollen mother cells. Their method of formation shows 

 them to be microspores, and the pollen sac is consequently a 

 microsporangium and the stamen a microsporophyll. The nuclear 

 and cell divisions within the pollen grain are stages in the germi- 

 nation of this microspore to form the male gametophyte. 



G. The carpellate cone at the time of pollination and its scales. 

 These cones appear on the new growth from the terminal 

 buds in the late spring at the same time as the staminate 

 cones. They are borne singly or in groups of two or three at 

 the tips of branches. Draw a habit sketch of the carpellate 

 cones on the new growth. 



1. Draw a side view of the carpellate cone, showing the 

 spiral arrangement of the cone scales. Each scale is be- 

 lieved to be a group of fused carpels or megasporophylls. 



2. Detach a cone scale carefully and draw it viewed from the 

 inner face. Note (a) the two ovules at either side of its 

 base, each with two horn-like appendages ; (b) a point on 

 the cone scale above the ovules and between them. 



3. Draw a side view of the cone scale, noting a small bract 

 in the axil of which the cone scale is borne. 



The ovule is a megasporangium with a protective envelope, the 

 integument, but the evidence for this conclusion can only be 

 understood after the study of sections of later stages (see J). 



H. The year-old carpellate cone and its scales. Search for year- 

 old cones, establishing their identity by their position on 



