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Practical Plant Biology. 



fleshy axis of the cone. On this axis above the membranous scales 

 just noted, the sacs containing the dust or pollen are crowded. 

 These sacs are roughly cylindrical with rounded ends; they are 

 fixed in pairs on the under side of special triangular scales called 

 stamens. The base of the stamen the apex of the triangle is 

 attached to the axis, while the broader distal end of the stamen 

 beyond the pollen-sacs is reflected upwards as a membranous flap. 

 The greater part of the stamen is thin and membranous but down 

 its middle line is a ridge of tissue which is traversed by a single 

 vascular bundle. This bundle runs down into the bundles of the 

 axis of the cone and unites with those of the supporting long shoot. 

 The pollen-sacs are attached to the under side of the stamen at 

 each side of the mid-rib containing the vascular bundle, their long 

 axis being parallel to the mid-rib. There are between sixty and one 

 hundred stamens in each pollen-cone and they are arranged on the 



FIG. 63. Pinus silvestris, stamen, end view on left, side view on right, x 25. 

 Side view shows pollen-sac dehiscing. 



axis in close spiral lines so that they completely cover it from view. 

 The upturned ends of the stamens cover the ends of the pollen - 

 sacs of the stamens above, while the yellow colour of the pollen - 

 sacs shines through their membranous substance. As the pollen-cone 

 matures a longitudinal slit develops in the wall of each pollen -sac. 

 This slit is formed by the contraction of the outer layer of cells in 

 the wall which, as water evaporates from them, are compelled by 

 their rigid internal supports to contract tangentially thus tearing 

 the wall longitudinally. Through this rupture the yellow dust 

 the pollen contained in the sac is released and falls on to the 

 upper surface of the stamens below. The pollen is retained there 

 by the upturned outer ends of these stamens. It is finally set free, 

 by the elongation of the axis separating the stamens, and is 

 launched on the wind by the swaying of the branches. 



The pollen coming from these cones is a uniform fine yellow 

 powder. Each grain is a rounded mass about 0-04 mm. in diameter. 



