428 A TEXTBOOK OF GENERAL BOTANY 
are particularly characteristic of north-temperate regions, where 
they form valuable forests of great extent. Conifers are fre- 
quently numerous on tropical mountains, and in limited areas 
may be the dominant element of the flora. 
CLASS ANGIOSPERMAE 
The angiosperms are the culmination of the evolutionary proc- 
ess in plants. In them the sporophyte has reached its greatest 
specialization, while 
the gametophyte has 
become greatly re- 
duced. They are 
the dominant ele- 
ment of our land 
flora, and in number 
of species exceed all 
other green plants. 
Fig.484. Megaspore mother cell and megaspores They include all the 
of an orchid woody plants other 

Left, megaspore within nucellus; center, megaspore than the gymno- 
is divided into two daughter cells; right, lower sperms and the tree 
daughter cell has divided into two megaspores, while ferns..and alae 
the upper one is in process of division. (Redrawn : 
after Brown and Sharp) ceous plants other 
than the ferns and 
fern allies. Many of the angiosperms have become specialized for 
aquatic conditions, and such species form the dominant vegeta- 
tion of fresh waters. Angiosperms have also invaded salt water, 
and even here grasslike forms may be the dominant element of 
the vegetation in rather quiet water where the bottom is muddy. 
The angiosperms are divided into two subclasses, the Dicoty- 
ledoneae (dicotyledons) and Monocotyledoneae (monocotyledons). 
The distinctive characters of these groups have been given in 
earlier chapters. The most primitive dicotyledons appear to have 
been woody ; it is believed that the herbaceous dicotyledons and 
the monocotyledons have been derived from woody dicotyledons. 
