96 SCIENCE PRIMERS. [xxiv 



membrane (Sect. 108), but does so from a group of 

 cells that form in its cavity. 



142. Their ovules are borne on the upper surface 

 of the scales of the female cone, which scales con- 

 sist of an open carpellary leaf seated upon and com- 

 bined with a bract (these are undistinguishable in the 

 Scotch fir, but distinguishable in the larch). They 

 resemble the ovules of flowering-plants, and like them 

 may have one or two coats, and be straight or inverted 

 through the unequal growth of one side (Par. 115). 

 The embryo-sac becomes filled with cellular tissue at 

 an early period. Within this tissue, beneath the upper- 

 most layer of cells, forming the top of the sac, several 

 larger cells appear, and form as many secondary 

 embryo-sacs. At the same time, that one cell of 

 the uppermost layer which lies immediately above 

 each secondary sac, divides longitudinally into four, 

 leaving a canal between for the passage of the pollen 

 tube. 



143. Fertilization takes place by a pollen-grain, 

 carried by the wind, alighting on the top of the nucleus 

 of the exposed ovule, and sending its tube through the 

 cellular substance of the nucleus, down to the primary 

 embryo-sac. There it reaches the passage between 

 the four cells above a secondary sac, traverses it, and 

 touches the latter. On this contact taking place 

 the contents of the secondary sac are divided by a 

 transverse partition into two portions, the lower of 

 which subsequently again divides and forms four 

 filaments that descend into the tissue of the primary 

 sac and nucleus. In the nucleus each filament begins 

 to form an embryo by cell-division at its extremity, 

 but only one embryo usually arrives at maturity. 



