The Plani Kingdom - 619 



MEGASPOROPHYLL 



MICROPYLE 

 INTEGUMENT 

 NUCELLUS 

 POLLEN TUBE 



ARCHEGONIUM 



MEGAGAMETOPHYTE 



Fig. 31-26. Above, one scale (megasporophyll) of a 

 pistillate pine cone, bearing two ovules. Be/ow, female 

 gametophyte (megagametophyte) at center of a de- 

 veloping ovule. Note the two male gametophytes (pol- 

 len tubes), growing toward the egg cells, in the arche- 

 gonia. The haploid gametophytes, both male and 

 female, are nurtured by the nucellus tissue and pro- 

 tected by the integument of the ovule while fertilization 

 occurs. Subsequently, the embryo of the new sporo- 

 phyte, which originates from the fertilized egg, is also 

 nurtured and protected while the ovule (as a whole) 

 is ripening into a seed. (From The Plant World.) 



tions, gymnosperms have occupied great areas 

 on all continents. 



The fossil evidence indicates that the 

 Gymnospermae originated from a now-ex- 

 tinct group, the "seed ferns" (Cycadofilicales), 

 which had a fairly wide distribution in the 

 Carboniferous period. Indeed, the cycads 

 (Fig. 31-27), which constitute the most primi- 

 tive group among surviving Gymnospermae, 

 show many resemblances to ancient "seed 



ferns." The sperm, among cycads, are flag- 

 ellated, even though they are carried to the 

 eggs by a pollen tube and never have an 

 opportunity to swim. Also cycads display 

 some other very primitive characteristics. 



Ginkgo biloba (Fig. 31-28), on the other 

 hand, represents the only surviving species 

 from an order (Ginkgoales) that previously 

 consisted of many genera and species. This 

 so-called living fossil, a native of China, has 

 recently become quite popular as an orna- 

 mental tree in the United States. 



The Angiospermae (Class 3; Phylum 

 Tracheophyta; Subphylum Pteropsida). 

 The angiosperms represent by far the largest 

 (250,000 species) group among the land 

 plants of today. Aside from conifers, most 

 plants in almost all terrestrial environments 

 are angiosperms. 



As was explained in Chapter 12, flowers 



Fig. 31-27. A cycad (Zamia). The cycads, a very 

 primitive group of gymnosperms, have flagellated 

 sperm cells. Note one large seed cone at the apex of 

 the short tuberous stem. (From The Plant World.) 



