368 



THE SPERMATOPIIYTES 



The outer bark is developed from the primitive or ground 

 tissue which lay outside of the circles of wood and bast when 

 these circles were first formed by the union of the primary 

 fibro-vascular bundles (Fig. 297, A), as described in Sec. 79. 

 There is much actively growing tissue in the bark, but the outer 



FIG. 298. The staininate cone, stamen, and pollen of the Scotch pine 



(Pinua sylvestris) 



A, young growth, with staminate cones about two weeks after the opening of the 

 terminal bud. B, details of cone. C, end view of stamen. D, side view of 

 stamen. E, pollen mother cell developing four pollen grains in a tetrad. F, 

 pollen grain showing the two wings: p, prothallial cell; g, generative cell; 

 t, tube nucleus. E, after Miss Ferguson 



regions become quite dead, and crack under the pressure of the 

 growing cambium, thus forming scales. The cracks are healed 

 by the living tissue of the bark. The bast is generally so closely 

 attached to the outer bark that it peels off with it, and therefore 



