CHAPTER XX. 



PHANEROGAMIA. 

 I. GENERAL CHARACTERS. 



496. In this Division the alternation of generations 

 which is so well marked in Bryophytes and in most Pterido- 

 phytes disappears. We have seen that in the higher Filicinae 

 and Lycopodinae there is a great reduction in the size and 

 importance of the prothalliuin (the sexual generation) ; in 

 EquisetacecB and Filices it is a large growth, which soon be- 

 comes entirely independent of the spore from which it origi- 

 nates ; in OphioglossacecB and LycopodiacecB it is of consid- 

 erable size, but it is less capable of leading an independent 

 existence ; in Rhizocarpece and SelaginellcB it is reduced to a 

 small outgrowth of the spore ; and in IsoetecB the reduction 

 is still greater, the small prothallium being little more than 

 the transformed spore contents. 



With the decrease in the structural importance of the 

 prothallium in these orders of the Pteridophyta, there is a 

 noticeable increase in the differentiation of the spore before 

 its separation from the parent plant ; thus in the three last- 

 named orders the spores have differentiated into (1) small 

 ones, microspores.which are strictly male as to their functions, 

 and (2) larger ones, macrospores, which are as strictly female. 



496. In the Phanerogamia the changes begun in the 

 Pteridophyta proceed a step further. The differentiation 

 into male and female organs of reproduction is carried back 

 far beyond the formation of the microspores (pollen grains) 

 and macrospores (embryo sacs) ; the macrospore does not 

 sever its connection with the parent plant, but continues to 

 be nourished by it until after the embryo is formed ; and as 



