THE ORIGIN OF LIFE 



tinguishable. This alternation of a diploid, sporophytic generation with a 

 haploid, gametophytic generation is one of the most fundamental features 

 of plant biology. Alternation of generations is first introduced with the 

 origin of sex, and it is present in all sexually reproducing algae; but in the 

 lower algae, the sporophytic generation is represented only by the zygote. 

 In Uha, and in many other green algae, however, the sporophyte is as 

 highly developed as is the gametophyte. 



Thus within the algae at large, and more especially within the green 

 algae, advances have been made which approach the condition of the 

 simpler vascular plants. The nucleus has been delimited from the cyto- 

 plasm by a membrane, and the mitotic mechanism has been perfected. 

 The grass-green chlorophyll is no longer masked by the blue phycocyanin, 

 and it is contained in chloroplasts like that of higher plants, not dissolved 

 in the cytoplasm as it is in the blue-green algae. The most primitive algae 

 are unicellular, but colonies of increasing complexity have been formed, 

 leading finally to true multicellular individuals. The simplest of these are 

 unbranched filaments, but these have given rise to branched and re- 

 branched plants, and finally to large, fleshy plants with considerable dif- 

 ferentiation of tissues. These may include root-like, stem-like, and leaf-like 

 structures. Cellulose walls are present in some of the algae. Also, a great 

 range of reproductive mechanisms has been developed among the algae. 

 The most primitive algae reproduce only asexually, either by simple fis- 

 sion, or by the formation of clusters of zoospores. Isogamy, the most 

 primitive form of sexual reproduction, probably began with the pairwise 

 fusion of undersized zoospores. Later, these gametes became differenti- 

 ated into small microgametes and large macrogametes, both of which 

 were motile (heterogamy). Finally, the macrogamete became a very 

 large, nonmotile cell which was sought by the microgamete. Meanwhile, 

 the alternation of generations was developed, with the diploid generation, 

 at first a minor incident in the life cycle, becoming increasingly prominent. 



The most advanced characters of the algae are all carried over into the 

 vascular plants. There is no single alga in which all of these characters 

 are present, yet it is clear that the trend of development in the green algae 

 especially is toward the type of organization characterizing the vascular 

 plants, and it is highly probable that the most primitive vascular plants 

 were derived from green algae. But the evolution of the vascular plants 

 and the bryophytes will be taken up in the next chapter. 



The events discussed in this chapter are undoubtedly among the most 

 important in the whole history of evolution. In point of time, they must 

 have occupied much the greater portion of the entire history of life. Yet 

 all of these events must have occurred long before the earliest known 

 useful fossils were formed. Thus it is altogether probable that decisive 

 fossil evidence on the problems discussed in this chapter will never be 

 obtained, and that these subjects must always remain speculative, even 

 though some inferences may be made with a fair degree of probability on 

 the basis of primitive or archaic organisms now living. 



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