PINES 279 
are represented by but two living genera, Araucaria and 
Agathis. These and other genera were represented in 
the Northern Hemisphere in Mesozoic and later periods. 
502. Modern Pines (Family Abietaceae). These may 
be illustrated by the common Scotch Pine (Pinus silves- 
tris), in which the microsporophylls are 
massed into cones 1 centimeter long, and 
these cones are themselves massed in clus- { 
ters. Each microsporophyll bears two spor- 
angia on its lower surface. The microspores 
are spheroidal but the outer layer of the ,Fis- 255.— 
inus (micro- 
wall is often swelled out into two bladder- poeta Neaay 
like distentions at opposite sides. These 
microspores (‘‘pollen’’) escape from the sporangia in 
the spring, and may be carried by the wind for long 
distances (sometimes for hundreds of miles). 
503. The megaspore cones grow singly near the ends 
of the upper twigs of the season’s growth, and are about 
1 centimeter long. They consist of an 
axis on which are borne flat megasporo- 
phylls, each bearing two inverted mega- 
sporangia (ovules). In these plants fertili- 
zation is a slow process: the microspores 
Fic. 156.—Pinus carried by the wind fall between the meg- 
(seed-eone). ~— asporophylls (in the spring or early sum- 
mer), where each spore pushes out a tubular antherid 
(“pollen tube’’) which penetrates the ovule tissue. This 
stimulates the growth of the tissues of the cone and it 
increases in size and bends downward on its stalk. In 
the meantime the ovules enlarge, the upper (‘‘chalazal’’) 
end developing a thickened mass of green tissue which 
grows far beyond the end of the sporophyll, constituting 
the “‘seed scale.’”’ These green “seed scales” are in 
