LIFE PROCESSES AND ORGANIZATION 



87 



parental cell. This is the reproductive process found 

 in many "single-celled" organisms, including some 

 algae and protistans. Mitosis also is the process lead- 

 ing to cell increase and to growth in multicellular 

 life. The consequence of mitosis is two cells, each 

 with a gene composition identical with that of the 

 parental cell. 



Amitosis probably has the same consequence as 

 mitosis; hence the process from a hereditary point of 

 view may be identical to mitosis. However, amitosis 

 is recognized because it is fission in which there is 

 segregation of nuclear material rather than structures 

 that can be recognized as chromosomes. Amitosis is 

 the typical reproductive process in the Monera. 



Fragmenlalion is the breaking apart of a multi- 

 cellular organism in such a way that each segment 

 can grow into a small replica of the adult. The 

 process of growth from a fragment is called regenera- 

 tion and is found throughout the plant kingdom, but 

 it is most typical of the simpler structured plants and 

 animals. In general, the less complex an organism, 

 the less strict are the requirements on the nature 

 of a fragment for regeneration to follow. 



Budding. Budding is an unequal division in which 

 an ofl'spring or a structure that can develop into an 

 offspring grows from the surface of a parent, dif- 

 ferentiates, and finally separates from the parent. 

 In the case of the offspring budding, the offspring is 

 a small replica of the parent. In the case of the 

 structure, growth and differentiation are necessary 

 to produce a miniature parent. 



Budding of miniature offspring occurs throughout 

 the protists and plants and in some animals, es- 

 pecially coelenterates, ectoprocts, entoprocts, ptero- 

 branchs, and tunicates. In the protists, because most 

 are unicellular, budding often is a kind of fission. In 

 most plants and animals budding ofTspring may be 

 best treated as a specialized kind of growth. It is a 

 mitotic phenomenon in which all cell divisions con- 

 tribute to the formation of a single daughter indi- 

 vidual that during development is a parasite upon its 

 parent. 



The formation of a true bud, a single cell, occurs 

 mostly in higher plants. A true bud is limited to an 

 asexual reproductive cell that develops directly into 

 an adult. Unfortunately, the term "bud" often is ap- 

 plied to early stages of miniature offspring budding in 

 animals and to certain multicellular plant structures 

 like the gemma of bryophytes, flower buds, and leaf 

 buds. Multicellular animal buds are found in sponges 



(gemmule) and ectoprocts (statoblast) and also grow 

 into a miniature adult. Gemma give rise to an indi- 

 vidual identical with the parent. Flower and leaf 

 buds are not asexual reproductive structures. Rather, 

 they are developmental flowers or leaves. 



The single-celled or true bud usually associated 

 with plants is called a spore. However, this true 

 spore is quite unlike most other true buds. Other 

 buds usually reproduce an individual that is a copy 

 of the parent, but spores produce entirely difTerent 

 individuals. A spore forms from an adult called a 

 sporophyte and typically grows into an entirely dif- 

 ferent looking adult called a gametophyte. More pre- 

 cisely, a sporophyte is a spore-producing organism 

 which alternates with a sex cell- (gamete-) producing 

 organism, a gametophyte. The occurrence of two 

 kinds of adults that regularly are of unlike appear- 

 ance often is difficult to appreciate. Although many 

 errors are introduced by the analogy, think of the two 

 adults in the life cycle of a single species as being 

 comparable to a worm and a human. In other words, 

 in such a life cycle an adult worm would produce a 

 bud that would grow into an adult human and a man 

 and woman would produce a fertilized egg that 

 would develop into a worm. The incidental error 

 that might be created by an extension of this analogy 

 can be corrected when the kinds of life cycles, par- 

 ticularly the generalized life cycle, are examined. 



Asexual Organismic Reproducfion. When restricted 

 to organisms, asexual reproduction is of two types, 

 vegetative reproduction and sporulation. Vegetative 

 reproduction includes all asexual types except that 

 involved in the formation of true buds, sporulation. 

 Therefore, vegetative reproduction includes fission in 

 the Monera and Protista, the budding off of entire 

 organisms or multicellular structures in plants and 

 animals, and fragmentation of plants and animals. 

 Fragmentation in certain plants, especially bryo- 

 phytes, arises from decay of parts between groups of 

 interconnected adults, the interconnected adults 

 having originated by budding. 



SEXUAL REPRODUCTION 



Sexual reproduction was defined as the process 

 dependent upon the fusion of two parental nuclei, 

 each from a difTerent and specialized sex cell called 

 a "gamete." In addition, each gamete contains a 

 single chromosome set (n) and nuclear fusion results 

 in a fertilized egg, or zygote, having two sets of 



