204 Plafit Biology 



contains many microspore mother cells which produce many micro- 

 spores (pollen grains). When still within the microsporangia, the nuclei 

 of the pollen grains divide to form two, one of which produces a prothal- 

 lial cell and one of which divides later to form ( 1 ) a generative cell and 



(2) a tube cell. These three-celled pollen grains (immature micro- 

 gametophytes) are carried by the wind to the female cones where, be- 

 cause of a sticky liquid, they pass through the micropyle of the ovule to 

 contact the nucellus. The tube cell forms a branched pollen tube through 

 the nucellus by digesting the latter. The generative cell divides to form 

 (1) a body cell and (2) a stalk cell. The body cell divides to form two 

 large multiflagellated (ciliated), m,otile sperms (antherozoids) . This 

 unique characteristic of wind-pollinated plants is probaby an ancestral 

 trait no longer needed. 



About six months after pollination a sperm fuses (fertilizes) with an 

 egg in the archegonium, forming a zygote. The latter develops ( 1 ) a 

 multicellular em.bryo, (2) two cotyledons (primary, embryonic leaves), 



(3) hypocotyl, (4) epicotyl, and (5) a long, coiled suspensor to push the 

 embryo in contact with the endosperm (food) of the megagametophyte. 

 Thus the original ovule is changed into a seed with its seed coat. Upon 

 germination the seeds develop either a staminate or a carpellate sporo- 

 phyte. Certain portions of the stem contain starch and may be used as 

 food. 



QUESTIONS AND TOPICS 



1. List the distinguishing characteristics of the subkingdom Embryophyta, the 

 phylum Tracheophyta, the subphylum Pteropsida, and the class Gymno- 

 spermae. 



2. Learn the meaning, pronunciation, and derivation of each new term used in 

 this chapter. 



3. Discuss the similarities between the conifers and the cycads. Discuss the 

 ways in which they differ. 



4. In what ways are gymnosperms considered to be higher plants than the ferns ? 



5. List the ways in which gymnosperms are of economic value. 



6. Make a detailed diagram of the life cycle of the pine tree, including the 

 various stages in correct sequence, with each labeled properly. 



7. Do you consider the conifers to be higher or lower plants than the cycads ? 

 Why? 



8. Considering the method of sexual reproduction in Zamia, do the sperm need 

 to be motile (flagellated) ? What explanation can you give for their motility? 



9. In the conifers, the sperm lacks flagella. Of what significance is this? 



10. Explain the phenomenon of alternation of generations in gymnosperms. 



11. Explain the formation and function of the pollen tube. 



