STEPS IN DEVELOPMENT 317 



MYXOPHYCEAE, contributed first of all the cell unit, to which 



they added a definite nucleus, and definite plastids. 

 CHLOROPHYCEAE, carried the plant body from the single cell 



to the rooted, branched filament, 

 added ciliated gametes, 



carried generation homisogamy to heterogamy, 

 carried the result of fertilization from the simple 



zygote to the simple fruit. 



BRYOPHYTA, 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). 

 PTERIDOPHYTA, 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). 

 CYCADOPHYTA, 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- 



sporophylls), 



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. 



