680 



TEXTBOOK OF BOTANY 



Seed plants are by no means restricted to land habitats; but if aquatic 

 they are generally confined to shallow waters of swamps and marshes 

 and along shores of lakes and rivers. They never grow in the open 

 waters of oceans or deep parts of lakes. Many flowering plants cannot 

 survive in deserts, but a few species grow in the most arid regions on 

 the continent. They are absent from polar areas and alpine heights 

 that are covered by ice and snow most of the year (Fig. 321). 



Fig. 321. Timberline on Mt. Edgecumbe, near Sitka, Alaska. Note valleys and 

 avalanche tracks. Below timberline is the hemlock-Sitka spruce forest; above is 

 the tundra. Altitude of mountain, 3,465 ft. Alaskan Aerial Survey Expedition, 1929. 

 Photo from U. S. Forest Service. 



The spermatophytes may be distinguished from all other plants by 

 the production of pollen tubes and seeds. They are either gymnospemis 

 or angiosperms. The seed of the gymnosperm is usually not enclosed by 

 any tissue corresponding to the carpel, and is said to be exposed or 

 naked. The seed of the angiosperm on the other hand is surrounded by 

 carpels.^ 



The gymnospemis are represented by the palm-like cycads ( Fig. 322 ) , 

 the Ginkgo (Fig. 323) with its dichotomously veined leaf, the shrubby 

 Ephedra, the shrubby or vine-like Gnetum, and the more common 



^ The so-called berries of cedars, yews, and arbor \'itae which are gymnosperms super- 

 ficially resemble fruits. Certain buttercups which are angiosperms have seeds incompletely 

 enclosed by carpels. 



