20 METASPERMAE OF THE MINNESOTA VALLEY. 



based upon the presence or absence of sexuality. We have, 

 therefore, the two great divisions: 



A. PROTOPHYTA: Plants in which sexuality has not been 

 developed and in the ancestral line of which it is believed, from 

 collateral evidence, that there are no sexually complete 

 progenitors. 



B. METAPHYTA : Plants which manifest sexuality or indicate 

 by accessory characters that in their ancestral lines there have 

 occurred sexually complete progenitors. 



These two great divisions are not clearly delimited, owing to 

 the presence of transition-forms which unite the lower group 

 with the higher. Such a form is the well known Ulothrix zonata 

 in which certain cells function indifferently as spores or 

 gametes (marrying cells). Furthermore, the limits are ob- 

 scured by such reduced forms of the Metaphyta, as undergoing 

 retrograde metamorphosis, have lost their sexual characters 

 and often resemble closely the upward-tending types of the 

 Protophyta, which are acquiring sexual characters, or on the 

 point of acquiring them, one might say. Such intermediate 

 forms, whether rudimentary or reduced, render exact limita 

 tion of the two great divisions quite impracticable. 



In similar fashion it is possible to arrange the Metaphyta in 

 two subdivisions based upon the development of the fertilised 

 egg. In the lower forms, after fertilisation, the egg proceeds 

 to develop a plant like the parent, which produced the egg; in 

 the higher forms, the egg undergoes a preliminary subdivision, 

 the result of which is the ultimate development of few or very 

 many cells, each of which is normally capable of producing a 

 plant like one of the parents. We therefore have the two fol 

 lowing subdivisions: 



I. GAMOPHYTA: Metaphyta which normally develop sexual 

 plants from their fertilised eggs without the interpolation of 

 any spore-producing structure. 



II. SPOROPHYTA: Metaphyta which normally subdivide 

 the fertilised egg into a cellular structure, capable of growth, 

 all or part of which consists, when mature, of spores, from 

 which sexual plants are normally produced. Such a cellular 

 structure is called a sporophyte or sporophytic plant. 



Examples of I. are the lower Zygophyta and Oophyta of Bessey 

 (32), plants like the pond-scum (Zygnema) or the black-mould 

 (Rhizopus, Mucor): examples of II. are too numerous to men- 

 tion, for in this subdivision are all plants inclusive of, and higher 



( 32) Bessey: Text Book of Botany, 6 ed. (1889) . 



