620 - Heredity and Evolution 



Fig. 31-28. The ginkgo tree (Ginkgo b//oba). It is sometimes cited as a living fossil because it is 

 the only species that has survived from a large group (the Ginkgoales) of primitive ancient gymno- 

 sperms. (From The Plant World.) 



(rather than cones) are distinctive of the An- 

 giospermae. Also the seeds of the angiosperm 

 aye enclosed within a fruit, which is a deriva- 

 tive of the basal part of the pistil (ovule 

 case). 



endosperm tissue (p. 221); (4) the growth of 

 the male gametophyte (pollen tube) is sup- 

 ported by the tissues of the upper parts of 

 the pistil (stigma and style), rather than by 

 the tissues of the ovule itself; and (5) the 



The life cycle of the angiosperm (Fig. 12- archegonia of the female gametophytes are 



19) is exceedingly well adapted to land con- reduced even further (in fact they are virtu- 



ditions. The basic pattern is the same as in ally absent). 



the Gymnospermae, except that (1) the The sporophyte generation among angio- 



gametophytes are even more reduced (Fig. sperms varies tremendously from lowly 



31-29); (2) the microspores (pollen) are car- grasses to mighty oak trees. Generally speak- 



ried mainly by insects (rather than by wind); ing, however, the tissues and organs are very 



(3) one of the sperm nuclei participates in an well adapted to land conditions. The xylem 



accessory fertilization, which forms a triploid almost always displays well-developed ducts, 



