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chapter 21 (dibiontic life cycle (monobiontic (interpolation hypothesis…

- - - - postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically. the sporophyte generation would have gradually evolved in complexity while the gametophyte generation remained small.
      - transformation theory
        
        postulates that after the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex, and vascularized, in a life cycle with alternation of isomorphic generation.
        
        equal dichotomous branching
        
        both branches being of equal size and vigor. plants of cooksonia had an epidermis with a cuticle, a cortex of parenchyma, and a simple bundle of xylem composed of tracheids with annular secondary walls.
        
        rhyniophytes
        
        as in all plants, only the central cells were sporogenous, and the sporangium had to open to release the spores. plants of cooksonia were homosporous; there were no separate micro-spores and mega-spores. fossils that have these general characters
        
        protostele
        
        early vascular plants had tow types of xylem organizations. in both, the center is a solid mass of xylem with no pith
        
        endarch protostele
        
        protoxylem is located in the center and meta-xylem differentiates on the outer edge of the xylem mass. protoxylem is the xylem that differentiates while cells are small and narrow, and meta-xylem differentiates after the cells have expanded for a few more hours or days and are larger.
        
        exarch protostele
        
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