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TEXTBOOK OF BOTANY 



conifers, such as the pine, spruce, hemlock, cedar, sequoia, and aura- 

 caria. No herbaceous gymnosperms are known. The angiosperms may- 

 be herbs, shrubs, or trees, and are classified as monocots and dicots. The 

 gymnosperms will be described first. 



The Gymnosperms: Conifers 



The gymnosperms are woody perennials, and nearly all of them are 

 evergreen. Of the approximately 500 species, about 350 are conifers, 

 among which are the oldest and largest of living plants. They are 

 widely distributed from the tropics to the tundra border and the tree 

 line on mountains. In contrast to the pteridophytes they bear seeds, have 

 deep and well-developed root systems, much-branched stems, and ex- 

 tensive leaf surfaces. The buds of conifers are scale-covered; they may 

 be dormant or grow similarly to those of angiosperms. The pine may 

 be used to exemplify the life cycle of the group. 



The pine sporophyte. The pines are generally recognized by their ex- 

 current branching, needle-like leaves usually in clusters of two to five 







Fig. 324. Cross and longitudinal sections of an Austrian pine needle. An 

 endodermal sheath one cell thick surrounds the centrally located veins. The 

 chlorenchyma, in which several resin ducts occur, is composed of peculiarly lobed 

 cells. The stomates are in pits, which are bordered by sclerenchyma, located just 

 beneath the epidermis. Section B was cut in the plane a-b of section A. 



on dwarf branches, and their evergreen habit. Leaves may remain on 

 the tree from 3 to 15 years, but their photosynthetic value decreases 

 rapidly with age. The linear leaves have a thick cuticle, sunken stomates. 



