THE SEED PRODUCTION OF PlNUS SYLVESTRIS. 17 



of the winter season, but now, responsive to the warm 

 sunbeams, the cells renew their marvellous activities of 

 growth, multiplication, and function ; they have much to 

 do, for in three and a half months the small cone of some 

 10 mm. in length has to be built up into one of over 

 50 mm. in length. 



About the 25th of March cell-division is taking place 

 in all the various tissues, and one or two mitoses of the 

 free nuclei of the gametophyte may take place before the 

 1st of April. These nuclei are 10 microns in diameter, 

 and contain at this period one nucleolus ; they are entirely 

 parietal, being embedded in the thin cytoplasmic layer 

 forming the inner lining of the young gametophyte. This 

 layer, in a preparation, is approximately about the same 

 thickness as the diameter of the free nuclei, viz., 

 10 microns ; in the living condition it is much thicker. 



The diameter of the gametophyte in the last week of 

 March is about 150 microns. Owing to its delicate wall 

 and large unsupported cavity it is difficult to secure a 

 preparation free from collapse ; osmic or corrosive I have 

 found useful as fixing agents during this early period. 



The tissue surrounding the gametophyte is very lax, 

 with exceedingly thin and tender walls ; its tapetal 

 character at this period is very evident ; its cells are 

 dividing, and its extent is increasing rapidly. The nuclei 

 of these cells are 13 microns in diameter, and when ripe 

 for division are 22 microns. The pollen tubes have com- 

 menced a very slow growth into the nucellus, the cells of 

 which are charged with an abundant food supply in the 

 form of starch. The generative cell has grown to a 

 diameter of 35 microns, its nucleus being 15 microns 

 diameter : it is now preparing for division. This takes 

 place during the second week of April ; the resulting cells 

 when first cut oif are usually alike in size and appearance, 



c 



