LIFE PROCESSES AND ORGANIZATION 



89 



may be internal, within the parent's body, or ex- 

 ternal, in the medium. As a general rule, external 

 fertilization is typical of aquatic life and internal of 

 terrestrial life, but there are exceptions. The marine 

 squids, octopuses, and barnacles carry on internal 

 fertilization even though they are aquatic, and in 

 aquatic salamanders, the male lays a packet of sperm 

 on the substrate, the female picks it up, and 

 fertilization is internal. 



LIFE CYCLES 



The history of the developmental, reproductive, 

 and any other stages that exist in an organism is 

 called a life cycle or life history. For convenience, 

 life cycles may be segregated into two types, asexual 

 and sexual; however, there are different kinds of 

 each, and both types can be united in the life history 

 of a single species. Also, there is no general kind of 

 life history that is unique to any of the kingdoms. 



The various life cycles are related to adults of 

 two chromosomal types. Adults may have one set of 

 chromosomes (symbol n and called haploid) or two 

 sets of chromosomes (symbol 2n and called diploid). 

 It will be seen that there is no over-all direct relation- 

 ship of either meiosis or mitosis with sexual or 

 asexual reproduction. 



Although many kinds of life cycles are recognized, 

 all can be derived from a single generalized, but 

 probably not the ancestral, cycle that includes a 

 sexual and an asexual phase. In all these life histories 

 the various stages have one or two sets of chromo- 

 somes, norln. The presence of the haploid or diploid 

 condition is organized so cycles might have both a 

 haplophase and a diplophase, or, perhaps, only one of 

 these phases. When both phases occur, only one may 

 have an adult {haplobionlic) , or each phase may include 

 an adult {diplobionlic) . In addition, a life cycle might 

 possess one or more stages where asexual reproduc- 

 tion occurs and brings about either the duplication of 

 the stage having such reproduction or the production 

 of an earlier or later stage in the cycle. If these varia- 

 tions are kept in mind, one sees that the accompany- 

 ing figure is no more than a generalized life cycle 

 (Figure 6.4). 



In Figures 6.4 through 6.7 at least part of each life 

 cycle indicates vegetative reproduction. The fact that 

 this reproduction normally is not an essential step in 

 the life cycle is indicated by its starting from and end- 

 ing at the same stage of the life cycle. The only place 



DIPLOBIONTIC LIFE CYCLE 



Diplophase ................... 



meiosis 



Haplophase 



::::(n)" 



veg. reprod.wadult 

 :•:•:•:•:•:•:•:•:•:•:•:•:•:■:•:•:• zygote; 



gametes 

 :-:-:-:-:-:':-:':-:-:fertilization . • • • 



Figure 6.4 The generalized, or diplobiontic, life cycle. 



where it always is essential is in the asexual cycle 

 (Figure 6.7). 



So far we have considered asexual reproduction 

 only as a secondary part of a given life cycle. We 

 have treated it as secondary in the sense of not men- 

 tioning it in the inner circle of a cycle. However, this 

 need not be the case. The production of spores is an 

 asexual process and usually is primary because it 

 often is limited to the inner circle of a cycle. There- 

 fore, primary asexual reproduction generally is by 

 sporulation and secondary asexual reproduction gen- 

 erally is by vegetative reproduction. 



DIPLOBIONTIC CYCLE 



This generalized life cycle is diagnosed by the two 

 diflferent, n and 2n, adults, the reason for its char- 

 acterization as diplobiontic. The other term for the 

 cycle, diplohaplonlic , also refers to the diploid and 

 haploid adults. Such a cycle contains a distinct alter- 

 nation of generations, because there are a sexual and 

 an asexual phase and the two kinds of adults often 

 are distinguishable from one another. Diplobiontic 

 cycles are most characteristic of plants; in the plant 

 kingdom the diploid adult is the sporophyte and the 

 haploid adult is the gametophyte. 



Note that meiosis does not produce gametes. In 

 plants having sporophyte and gametophyte genera- 

 tions, the diploid sporophyte forms haploid spores by 

 meiosis. These spores germinate by mitosis into a 

 sexual generation, the haploid gametophyte. The 

 gametophyte produces haploid gametes by mitosis. 

 After gamete fusion (sexual reproduction) and diploid 

 zygote formation, mitosis again is involved in develop- 



