Two sperm nuclei 



Growth nucleus' 



Hugh Spencer 



POLLEN TUBE 



Under suitable conditions, pollen grains sprout like spores, the protoplasm growing 

 out into a long thread. The haploid nucleus in the pollen divides into two. One of 

 the nuclei seems to direct the growth of the tube. The other divides again: these 

 final nuclei are the true sperm, or male, cells 



How Does Fertilization Take Place in a Flower? 



The Meeting of Gametes^ In most common plants stamens and pistils 

 are borne in the same flower. Fertilization is nevertheless brought about in 

 a very roundabout way. The embryo sac remains inside the ovule, as the ovule 

 remains inside the ovary. All the traveling is done by the pollen. 



When the pollen grain alights upon the stigma of a pistil, it absorbs some 

 of the fluid on the latter. Then a very thin thread of protoplasm grows out 

 of the pollen grain — the "pollen tube". It is comparatively easy to get pollen 

 grains of many different kinds to sprout their pollen tubes in a drop of sweet- 

 ened water, on a microscope slide, and to observe some of the changes that 

 take place (see illustration above). 



The pollen tube normally grows through the style of the pistil into the hol- 

 low of the ovary. Then it grows through a small hole in the ovule that reaches 

 toward the embryo sac (see illustration opposite). Pollen tubes appear to be 

 chemotropic. When the tip comes in contact with the embryo sac, the cell- 

 wall melts away, and the two sexual nuclei combine. This is the essential fact 

 of fertilization. The zygote, having the double, or diploid, number of chromo- 

 somes, is the first cell of a new individual. It corresponds to the fertilized egg 

 of a fern or moss — or, for that matter, of an animal. 



The New Individual^ After fertilization, the mass in the embryo sac 

 absorbs food from the parent plant and grows into an embryo (see illustration 

 opposite). The surrounding walls of the ovule become the seed coats. The 

 ovule, with its embryo sac, thus changes into a seed. In addition to the 



iSee No. 2, p. 414. 2 See No. 3, p. 415. 



404 



