194 Plant Biology 



sporophylls are located belovv^ but on the same cone. Since Selaginella 

 produces two kinds of spores (microspores and megaspores), it is hetero- 

 sporous. 



A me gas pore germinates to form a me gagameto phyte (female game- 

 tophyte) within the megaspore while still in the megasporangium. As 

 the megagametophyte develops, it forms several female archegonia, rhi- 

 zoids, stored foods, and chlorophyll. 



A microspore develops within the microsporangia to form a small, 

 parasitic microgameto phyte (male gametophyte) . The latter is sur- 

 rounded by the microspore wall and consists of one prothalial cell and 

 one male antheridium. No chlorophyll is formed. The antheridium 

 produces hiciliated sperms. 



When the microsporangial walls rupture, the microspores are carried 

 to the megasporangia. The sperm swims to the archegonium where the 

 egg is fertilized to form a zygote. By cell division, the latter forms two 

 cells, the upper one becoming a suspensor to push the embryo into con- 

 tact with the stored foods of the megagametophyte. The other cell of 

 the zygote develops into the embryo. The latter develops a mass of cells 

 known as the foot by which foods are absorbed from the megagameto- 

 phyte. Eventually, the embryo produces a stem, root, and two cotyledons 

 (embryonic "seed" leaves). Later this young sporophyte becomes inde- 

 pendent by photosynthesizing its own food. The production of a sus- 

 pensor by the zygote is somewhat similar to a phenomenon in the higher 

 seed-producing plants. 



HORSETAILS (SCOURING RUSHES) 



Equisetum (ek wi -se' turn) (Gr. equus, horse; seta, tail). — The horse- 

 tails belong to the subkingdom Embryo phyta; phylum Tracheophyta; 

 subphylum Sphenopsida (sf en -op' si da) (Gr. sphen, wedge; opsis, ap- 

 pearance) because of the wedge-shaped leaves of certain species. All 

 horsetails belong to the genus Equisetum (Fig. 49) which has the follow- 

 ing characteristics: hollow, jointed stems, usually ribbed and containing 

 silica, branches and small leaves (sometimes scalelike) in whorls, and 

 strobili (cones) composed of whorls of shield-shaped sporangiophores, 

 each of which bears five to ten sporangia (spore cases) . 



The sporophyte of Equisetum consists of a branched horizontal rhi- 

 zome with nodes. The latter bear whorls of scalelike leaves. In most 

 species, the rhizomes bear ( 1 ) upright, colorless, unbranched fertile stems 

 with a strobilus at each tip and (2) upright, green, bushy vegetative 

 (sterile) stems with many whorled branches at the nodes. Because of 



