REPRODUCTION IN PLANTS 109 



So far it is clear that a flower is a group of sporophylls 

 which produce microspores and megaspores. Since, how- 

 ever, such reproductive bodies always form male and female 

 gametophytes, their development must now be considered. 



The first fact to have clearly in mind is that the megaspore 

 is never liberated by the megasporangium. And further that 

 the latter remains just where it arose in the ovule case of the 

 pistil. Consequently the megaspore germinates within the 

 pistil, and it forms there a female gametophyte composed of 

 only a few cells, including the female gamete, or egg. Thus 

 the female gametophyte generation of Flowering Plants is in- 

 visible except with the microscope. 



The pollen grain is a typical microspore, a single cell en- 

 closed within a protective wall. Germination starts, while 

 the pollen is still in the anther, by the division of the spore 

 nucleus into two, one of which divides again. Further devel- 

 opment does not occur unless the pollen is transferred in 

 some way, usually by insects or the wind, to the stigma of 

 the pistil. The stigma secretes fluids suitable for the germina- 

 tion of the ripe pollen grain, which bursts its rigid wall and 

 puts forth a cytoplasmic tube. This grows down through the 

 tissues of the pistil until its tip enters the ovule case, and 

 carries with it the nuclei, two of which represent sperm. The 

 pollen has now completed its development and thus the con- 

 tents of the pollen grain plus the tube itself constitute a 

 greatly reduced male gametophyte. 



By the time the pollen tube reaches the ovule case, the 

 megaspore within has formed, as already described, the 

 female gametophyte with its egg. One of the sperm nuclei 

 unites with the egg and forms a zygote, which remains just 

 where it is, surrounded by the tissues of the pistil base, and 

 proceeds to divide to form an embryo sporophyte with rudi- 

 mentary root, stem, and leaf. Concurrently, the ovule case 



