208 - Multicellular Planfs 



Haploid Phases 



Fertilization 



Diploid Phases 



-*- Zygote 



Egg Sperm 

 Gametophyte 



Sporophyte 



Spores •*- 



Meiosis 



Fig. 12-7. Generalized life cycle of a typical higher plant. 



The Cyanophytes (Blue-Green Algae). The 



blue-green algae (see p. 594) are very primi- 

 tive plants that display no clearly defined 

 form of sexual reproduction. Generally they 

 reproduce by binary fission (p. 55), although 

 quite a few species also form spores. None is 

 truly multicellular; that is, all are either uni- 

 cellular or colonial. Essentially these primi- 

 tive forms are aquatic plants, although many 

 species live in moist places, such as damp 

 soil. No sharp distinction between nucleus 

 and cytoplasm has been developed in the 

 cells of a majority of these primitive organ- 

 isms. 



In the absence of sexual phenomenon, it 

 is not possible to speak unequivocally about 

 haploid and diploid stages, although by con- 

 vention the blue-green algae are regarded as 

 being haploid throughout their entire simple 

 life cycle. 



The Schizomycophytes, or Bacteria (Chap- 

 ter 31). These colorless unicellular or co- 

 lonial plants began to evolve even before the 

 blue-green algae. Nevertheless some bacterial 

 cells appear to possess one or more nuclei 

 or at least masses of nuclear material that 

 seem to be delimited within a membrane. 

 Moreover, some bacteria display a primitive 

 sort of sexual reproduction by which genie 

 material may be passed from one individual 

 to another, so that genie recombinations (p. 

 563) may occur. The sexual processes are 

 somewhat ambiguous, however, and it is not 

 possible to distinguish clearly between hap- 

 loid and diploid phases of the life cycle. 



Higher Thallophytes (Chapter 31). Many 

 of the higher Thallophyta possess chloro- 

 phyll (green, red, brown, and other algae); 

 but others (yeast, molds, rusts, smuts, mush- 

 rooms, and other fungi) lack this green pig- 

 ment. Some are truly multicellular although 

 many species remain unicellular or colonial. 

 The cells of the higher Thallophyta display 

 distinct nuclei; and practically all have de- 

 veloped some form of sexual reproduction 

 that supplements the asexual methods. But 

 a regular alternation of sexual (gameto- 

 phyte) and asexual (sporophyte) generations 

 is not generally characteristic of these plants. 

 Thev have remained essentially aquatic, al- 

 though many parasitic and saprophytic spe- 

 cies have been evolved. The body structure 

 is simple, without true roots, stems, or leaves 

 (thallus type of body). 



The Bryophytes (Chapter 31). The Bryo- 

 phyta, which include the mosses and the 

 liverworts, are among the simplest of the ex- 

 isting land plants. All the Bryophyta display 

 a regular alternation of sexual and asexual 

 generations. Among Bryophyta, however, the 

 gametophyte is the dominant form of the 

 plant, and the sporophyte is nutritively de- 

 pendent upon the gametophyte (see later). 

 Adaptation to the land is not very complete. 

 The mosses have developed a very simple 

 leaf and stem system, but possess rhizoids in 

 place of true roots. The liverworts, most 

 primitive of the Bryophyta, usually have a 

 thalloid body, devoid of any root, stem, or 

 leaf system (see Chap. 31). 



