422 PLANT MORPHOLOGY 



Among the gymnosperms a separation of microspore-bearing and 

 megaspore-bearing structures to two kinds of cones is another expression 

 of sexual differentiation. In the cycads, Ginkgo, and some conifers this is 

 extended to the entire sporophyte, so that there are male and female 

 plants as well as male and female cones. Most angiosperms bear flowers 

 having both stamens and carpels, but some have two kinds of flowers, one 

 with stamens and the other with carpels. The two kinds may occur on 

 the same plant or on separate plants, depending on the species. 



Significance of Sex. The most important feature of sexual reproduc- 

 tion is the union of the two gametic nuclei. This brings together two 

 haploid sets of chromosomes to form a diploid zygote nucleus. Each set 

 consists of innumerable genes that determine hereditary characters and 

 each ordinarily represents a somewhat different assortment of genes. 

 Meiosis, sooner or later following gametic union, provides a means of 

 reshuffling the paternal and maternal chromosomes brought together 

 in the previous act of fertilization, thus resulting in many new gene 

 combinations. 



It is evident that sexual reproduction, through fertilization and meiosis, 

 creates great variation among individuals related by descent and so fur- 

 nishes raw material for evolutionary processes to work upon. Asexual 

 reproduction results in organic similarity; sexual reproduction results in 

 diversity. The significance of sexual reproduction is not primarily the 

 multiplication of individuals, but the production of heritable variations 

 that accelerate the process of evolution. It is in this feature that its 

 great advantage lies. In plants first reaching the level of sexuality, this 

 advantage would tend to be perpetuated through natural selection and to 

 become established as a permanent part of the life history. 



ALTERNATION OF GENERATIONS 



Many algae and fungi, as well as all plants above the thallophyte level, 

 are characterized by an alternation of generations, in which the life cycle 

 consists of two kinds of individuals that follow each other in alternate 

 sequence. One of these, the gametophyte, is haploid and produces 

 gametes, while the other, the sporophyte, is diploid and produces spores. 

 The diploid condition arises in the zygote, produced by gametic vuiion. 

 The zygote develops into a sporophyte, meiosis taking place when spores 

 are formed. The spore gives rise to the gametophyte. The zygote is the 

 first cell of the sporophyte generation, and the spore is the first cell of the 

 gametophyte generation. 



Types of Life Cycles. Fertilization and meiosis are the two cardinal 

 events in every life cycle involving sexual reproduction and each is a 

 necessary consequence of the other. Thus every plant with sexual repro- 

 duction displays both a haploid and a diploid phase, but in many thallo- 



