6 HEREDITY AND EVOLUTION IN PLANTS 



that they are not all alike. Some of them are merely 

 foliage-leaves, and do not differ in any essential point from 

 the foliage-leaves of higher plants, such as the maple or 

 lily; they possess stomata for the exchange of gases and 



Fig. 7. — Portions of the sporophylls of two ferns to show the sori. 

 On the left Poly podium punclatum (L.) Sw. ; on the right a variety of Pleris 

 longifoUa, with sporangia marginal on the pinnules. 



water-vapor with the outer air (Fig. 5), and they also 

 resemble the leaves of higher plants in their internal struc- 

 ture. All fern-leaves, however, have a very characteristic 

 arrangement in the embryonic or bud condition, being 



