138 Plajit Biology 



A motile, multifiagellated sperm swims through water to fertilize the egg 

 (ovum) in the female archegonium, thus forming a zygote. The latter 

 produces a multicellular embryo which develops a sporophyte. A ger- 

 minating spore forms a small, thin, green, heart-shaped prothallus (pro- 

 thallium) with male antheridia and female archegonia. 



Examples: Pteridium (Fig. 50) and Poly podium (Fig. 51). 



B 



(9<S3 



Archeqomum 



\ /Antheridi ^ 



_ 5tGm 



0\/urr)\ 



Toot 



"Rhiioids 



Fig. 50. — Common fern (Pteridium) of the subphylum Pteropsida, class Fili- 

 cineae. A, Fern plant or sporophyte; B, sporangium enlarged and emitting spores; 

 C, two spores developing into prothalli; D, prothallus with archegonia (female) 

 near the notch with one archegonium enlarged to show the ovum ; antheridia 

 (male) near the rhizoids; E, similar prothallus with archegonia and antheridia, 

 with an enlarged antheridium to show the sperm; self-fertilization does not occur; 

 F, prothallus with a young fern plant growing out of the archegonium from the 

 fertilized ovum. The prothallus to which this sporophyte is attached eventually 

 will disappear. 



Gymnosperms. — The gymnosperms, or those plants which pro- 

 duce seeds exposed (naked) on female sporophylls, known as mega- 

 sporophylls, belong to the class Gymnospermae (jim no -spur' me) (Gr. 

 gymnos, naked or exposed; sperma, seed). The gymnosperms include 

 .the cone-bearing evergreens (conifers) and their allies. 



Gymnosperms are rather large, woody, perennial plants which are 

 mainly evergreen (retain leaves more than one growing season). Cer- 

 tain types may be short and shrubby. They possess true roots, stems, and 

 leaves; in the cone-bearing evergreens the leaves may be needlelike or 

 scalelike. The sporophyte generation is large, complex, and independent, 

 while the gametophyte is small (microscopic) and dependent (parasitic) 

 upon the sporophyte. Two kinds of cones composed of sporophylls are 



