EVOLUTION OF THE PLANT KINGDOM 417 



Spermatophytes. The spermatophytes reach the culmination of evolu- 

 tion in the development of the seed, development of pollen tubes, ultimate 

 enclosure of the seed by the carpel (angiospermy) , ultimate elimination of 

 swimming sperm.s and later of archegonia, a great reduction in the game- 

 tophyte generation, and the evolution of the flower. 



The abandonment of ciliated sperms by the conifers resulted in a change 

 in the pollen tube from a branched haustorial organ to a carrier of the 

 male cells. The male gametophyte has undergone a reduction in the 

 number of male cells to tw^o and of prothallial cells to two or one and 

 finall}^ to none. Throughout gymnosperms the female gametophyte 

 exhibits various stages in reduction that reach an extreme in angiosperms. 

 The tendency for eggs to mature earlier and earlier in the development of 

 the gametophyte has finally resulted in the elimination of archegonia. 



A marked feature of evolution in the seed plants has been the develop- 

 ment of the strobilus into a flower. Among angiosperms floral evolution 

 has been marked by a number of evolutionary tendencies, among which 

 are the following: floral parts numerous and spiral to few and cyclic, pen- 

 tacyclic to tetracyclic, free to united; perianth undifferentiated to differen- 

 tiated into a distinct calyx and corolla; corolla regular to irregular; flowers 

 hypogynous to perigynous and epigynous; ovules with two integuments 

 to only one; embryo dicotyledonous to monocotyledonous; endosperm 

 abundant to little or none; fruit simple to aggregate and multiple. Angi- 

 osperms also show trends from a woody to an herbaceous habit, from erect 

 types to vines, from perennials to annuals, from a spiral to a cychc 

 arrangement of leaves, from simple to divided leaves, and from net-veined 

 to parallel-veined leaves. 



It is clearly evident that the gymnosperms have been derived from the 

 pteridophytes and are a much older group than the angiosperms, whose 

 origin is obscure. If the angiosperms have not come from the gymno- 

 sperms, the presence of seeds, pollen tubes, and many other common fea- 

 tures are a result of parallel development. 



EVOLUTION OF SEX 



There are two primary types of reproduction in the plant kingdom, 

 sexual and asexual. The distinctive feature of sexual reproduction is the 

 fusion of two cells to form a zygote. In asexual or vegetative reproduc- 

 tion no such fusion occurs. 



Asexual Reproduction. Cell division is the simplest and oldest method 

 of reproduction and in many unicellular plants it is the only method. In 

 multicellular plants cell division does not result in reproduction but in 

 growth. To make reproduction possible, a cell or group of cells must 

 become detached from the parent plant. A spore shares with ordinary 



