GROUPS OF PLANTS. ^^ 



Gametophytes. — The development of the gametophytes 

 from the asexual spores, namely, the microspore or pollen grain, 

 and the megaspore or embryo-sac, is as follows : The nucleus 

 of the megaspore divides repeatedly (Fig. 50), cell walls 

 are formed and a multicellular structure known as the endosperm 

 is produced. This structure constitutes the prothallus of the 

 female gametophyte (Fig. 49, E; Fig. 50). In the upper portion 

 of the prothallus (that is, at the micropylar end), three to five 

 archegonia are formed (Fig. 49, a; Fig. 50), which are sepa- 

 rated from one another by cells of the endosperm or prothallus 

 which are rich in protoplasm. The structure of the archegonium 

 is much like that of the preceding group, consisting of a venter 

 which contains the ^g^, and a short neck composed of 4 to 8 cells. 



The male gametophyte begins to develop while the pollen is 

 still in the sporangium. At this stage it consists of a generative 

 cell and a wall-cell, which constitute the antheridium, the cells of 

 the prothallus being usually suppressed (Fig. 48, £)). 



In addition to the extreme minuteness of the gametophytes 

 we have also to note the character of the male gamete or sperm. 

 With the exception of the Cycads and Ginkgo, motile sperms are 

 not found in the Gymnosperms,' but these are represented by two 

 male nuclei which are transferred directly to the archegonium 

 from the male gametophyte, formed through germination of the 

 microspore (pollen grain). It may be recalled that in the Pteri- 

 dophytes the motile sperms are discharged from the antheridium 

 and carried by the agency of water to the archegonium, but in the 

 Gymnosperms water is no longer a medium of transferral. The 

 microspores themselves are carried to the ovules usually through 

 the agency of wind after which they germinate developing a tube 

 which carries the male nuclei directly to the archegonium without 

 their ever having been free. 



The transferral of the microspores or pollen grains to the 

 ovule is known as pollination. After pollination the wall-cell 

 develops a tube, the pollen tube, and the generative cell gives rise 

 to two male nuclei, which, with the remaining protoplasmic con- 

 tents of the antheridium, are carried by the pollen tube to the 

 micropyle, which it enters, penetrating the tissue of the nucellus 

 (Fig. 49, 0- O" reaching the neck of an archegonium the pollen 



