Lecture XXIII. 



199 



and roots. Some are also stored during the autumn and winter 

 in the medullary rays and next season may be thrown into the 

 tracheids to be transmitted to the various points where they are 

 needed for growth. The outer tissues cortex and bark act as 

 non-conductors to diminish the suddenness of external tempera- 

 ture changes, as waterproofing to isolate the bast from external 

 osmotic actions, and as armour to protect the inner tissues from 

 injury. At the same time the looseness of the cells in the cork 

 permits of an exchange of 

 gases through this coat- 

 ing which is necessary for 

 the respiration of the inner 

 cells, viz. those of the 

 cambium and medullary 

 rays. 



From this brief study of 

 the vegetative structure of 

 the Pine and cursory con- 

 sideration of the function 

 of its parts we must turn 

 our attention to its re- 

 production. 



Two kinds of cone are 

 found on the Pine which 

 are concerned in its re- 

 production : the dust- or 

 pollen-cones and the seed- 

 cones. They are both 

 borne near to the ends of 

 the branches. We will 

 begin with the examination 

 of the pollen-cones. 



The pollen-cones are 

 yellow in colour, they are 

 somewhat egg-shaped and 



FIG. 62. Pinus silvestris, pollen- cone, longi- 

 tudinal section, x 12. a, pollen-sac; b, 

 axis of cone ; c, conducting tract in axis; 

 d, outer end of stamen. (From Evans' An 

 Intermediate Textbook of Botany.) 



are about i cm. long. 



They are attached to the 



present year's growth by short stalks. A large number of them 



form groups on the base of the long shoot replacing lateral dwarf 



shoots. 



The stalk of the pollen-cone springs from the axil of a scale on 

 the long shoot. The stalk itself bears a number of overlapping 

 membranous scales, and is continued upwards as the thick, almost 



