GYMNOSPERMS 



309 



pass the cells formed by the division of the nucleus of the 

 microspore. This tube makes its way through the tissue of the 

 ovule, toward the egg (Fig. 252). The pollen tube really begins 

 to grow long before the egg is formed. Female gametophyte 

 and pollen tube both develop slowly, so that it is twelve or 

 thirteen months after pollination before the archegonium is 

 reached by the tip of the pollen tube. The tip of the pollen 

 tube opens and two of the cells that were carried down in it 

 pass out (Fig. 255). Either of them may unite with the egg 

 to produce an oospore. The other one disappears after a time. 

 These are the male cells. We should call them sperms, but 

 they do not have cilia, organs 

 of locomotion which are parts of 

 sperms. There are gymnosperms 

 (Fig. 263) which have real sperms 

 that are carried to the archegonia 

 by pollen tubes. The pollen tube 

 as a means of securing fertilization 

 is of great importance, and this 

 is shown in even a more striking 

 way in angiosperms. 



286. Embryo, seed, and seedling. 

 After fertilization, the oospore 

 develops into an embryo pine 

 plant. It is developed within the old female gametophyte from 

 which the developing embryo derives its nourishment. The 

 wall of the female gametophyte is now known as the embryo 

 sac wall. After a period of growth the embryo has developed 

 root, stem tip, and leaves, all closely packed within the female 

 gametophyte tissue. At this time the ovule wall becomes dry 

 and hard and the embryo stops growing. The whole structure 

 is now the seed (Fig. 256). 



When the pine cone opens, two years or more after the 

 spring when pollination occurred, the seeds fall from it. To 

 each seed is attached a wing, which sometimes buoys the 

 seed in the air, thus making wide distribution more probable. 



FIG. 256. Diagram of the seed 



of a pine showing the embryo 



(new pine plant) inclosed within 



the food material 



At the right tip of this embryo is 



the root, and at the left are the 



seed leaves which inclose the small 



stem tip 



