378 SCOTS PINE. 



528. T. S. of Male Cone. In a transverse section 



of the male cone, some of the stamens will be seen as T - 

 shaped structures, with a pollen-sac on either side of the T. 

 Some of the sacs in these sections will show the struc- 

 ture of the sac-wall; also mount in glycerine a single 

 stamen, and after noting the form of the stamen, flatten it 

 under a cover-glass, and note that the wall of the pollen- 

 sac shows numerous rod-like or U-shaped fibres in each 

 epidermis cell. 



529. Pollen-grains in various stages of development will have 

 been seen in the sections, lying in sifn in the pollen sacs. To trace 

 the stages, tease out the contents of pollen-sacs from (a) very young 

 cones still enclosed in the resting-bud in winter and early spring, 

 (?>) exposed cones taken at intervals during May and June. Note 

 that : 



(1) The pollen-grains are formed in fours (tetrads) from the 

 mother-cells. 



(2) The young grain is a spherical cell with a single nucleus. 



(3) On two opposite points the outer cell-wall layer (extine or 

 exospore) shows rounded projections containing sap and separating 

 it at these points from the inner layer (intine or endospore) of the 

 cell-wall. 



(4) The two projections grow until each is about as large as the 

 pollen-grain itself, and, as the cone dries, the sap in the projections 

 is replaced by air, so that the grain now bears two rounded balloon- 

 like vesicles or air -bladders. That they do contain air is easily 

 seen on mounting in water some grains from a dry cone and noting 

 the dark appearance due to the air, which is expelled from the 

 vesicles on irrigation with alcohol. 



(5) Each vesicle has its outer wall (extine) strengthened by a fine 

 network of thickenings. 



(6) The grain begins its germination before leaving the pollen- 

 sac, two small cells being successive^ cut off, each by a watch-glass- 

 like wall, from the convex end of the grain that farthest from the 

 vesicles. 



(7) These two male prothallus cells are soon obliterated, or 

 may be seen like cracks in the wall of the ripe grain, which shows 

 at this part an antheridial cell, cut off by a curved wall from the 

 larger cell which makes up the rest of the grain ; this larger cell is 

 often called the vegetative cell, or tube-cell, since it grows out 

 later to form the pollen-tube. 



530. Female Cone (General Characters). Examine 

 female cones of different ages. Note that in June cones 

 are found showing three stages of development, in different 



