STEPS IN DEVELOPMENT 317 
Myxopuycegag, contributed first of all the cell unit, to which 
they added a definite nucleus, and definite plastids. 
Cuioropuyceag, carried the plant body from the single cell 
to the rooted, branched filament, 
—added ciliated gametes, 
—carried generation fromisogamy to heterogamy, 
—carried the result of fertilization from the simple 
zygote to the simple fruit. 
Bryopuyta, developed the plant body as a cell mass, 
—developed the sporophyte from the simple fruit, 
and so brought in an obvious alternation of generations, 
and with it terrestrial life, 
with which came the beginning of supporting tissues 
(woody strands), 
and simultaneously the beginning of conducting tissues 
(vascular strands). 
PreripopHyta, reduced the gametophyte to a smaller and 
short-lived structure, 
—developed an independent sporophyte by the production 
of roots and leaves; 
—differentiated isospores into heterospores; (microspores 
and megaspores) ; 
—perfected the supporting tissues (woody strands); 
—perfected the conducting tissues (vascular bundles). 
Cycapopuyta, developed special sporophylls for megaspores 
(megasporophylls), | 
—retained the megaspore in the megasporangium, 
—which became covered by an indusium (integument), 
—reduced the archegonial gametophyte to a dependent 
structure retained by the megasporangium, 
—which led to the development of the seed, 
—developed special sporophylls for microspores (micro- 
sporophylis), 
—developed tubular antherids, 
—reduced the sperms to two, 
—aggregated the sporophylls into a cone (strobilus); 
—developed the beginnings of the perianth, 
—produced an erect, long-lived stem, 
—developed fibro-vascular bundles, 
and modes of thickening the stem. 
