PINES 279 



are represented by but two living genera, Araucaria and 

 Agatliis. These and other genera were represented in 

 the Northern Hemisphere in Mesozoic and later periods. 



502. Modern Pines (Family Ahietaceae). These may 

 be illustrated by the common Scotch Pine {Pinus silves- 

 iris), 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 p^^^^ 

 wall is often swelled out into two bladder- spore cone and 



microspore). 



like distentions at opposite sides. These 

 microspores C' 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 

 Fig. 156.— Pinus Carried by the wind fall between the meg- 

 see -cone). 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 doveloi)ing a thickened mass of grcMMi tissue which 

 grows far beyond the end of the sporojihyll, constituting 

 the ''seed scale." These green "seed scales'' are in 



