TBUE NATUBE OP PLOKAL OKGANS. 157 



through, the style (if there is one), there are found long cells 

 sometimes with intermediate spaces, through which latter the 

 pollen tubes readily find their way (Fig. 141). When no such 

 intercellular spaces exist, the pollen tube proceeds through 

 the cell walls, which it softens by means of a substance which 

 it exudes for that purpose. 



192. Structure of the Ovule. — The details of the micro- 

 scopic anatomy of the ovule are rather complicated. It is 

 enough for our present purpose to state that the young ovule, 

 before it has begun to form an embryo, usually exists as a 

 roundish or egg-shaped mass, with a small opening leading 

 into its apex. This opening leads to a sac inside the ovule, 

 filled with soft protoplasmic material, containing cells and 

 known as the embryo sac. Minute cells occur at the apex 

 of the ovule (Fig. 142), and it is from their growth and 

 development that the embryo is at length produced. 



