342 PLANT MORPHOLOGY 



Sporophyte. Almost all conifers are trees, but a few are shrubs. Typ- 

 ically they display the excurrent habit, with a tall straight trunk giving 

 rise to numerous wide-spreading branches. Nearly all conifers are ever- 

 green, retaining their leaves for from 3 to 10 years. The only deciduous 

 forms in North America are Larix and Taxodium. The largest conifers 

 are the two species of Sequoia, both native to California. Sequoia 

 sempervirens, the redwood of the coastal region, sometimes reaches a 

 height slightly in excess of 100 m., a diameter of 6 m.,^ and an age of 

 1,300 years. Sequoia gigantea, the big tree of the Sierra Nevada, attains a 

 maximum height of somewhat less than 100 m., a diameter of 8 m.,^ and 

 an age of about 3,500 years. This species is regarded by some botanists 

 as distinct enough to constitute a separate genus, Sequoiadendron. 



In almost all genera only stems of unlimited growth and with scattered 

 leaves are present. In Pinus, Cedrus, and Larix, however, both long and 

 dwarf (spur) shoots occur. In Pinus the long shoots bear scale-like 

 leaves and in the axil of each arises a spur shoot bearing needle-like foliage 

 leaves (Fig. 291). Only Pinus monophylla produces one leaf on a spur; 

 most species have either two or three leaves ; Pinus quadrifolia has four ; 

 and some species have five. In Cedrus and Larix foliage leaves are borne 

 both on the long and dwarf shoots and the number of leaves on the latter 

 is much larger than in Pinus, being usually 30 to 50. In Pinus the entire 

 spur falls away with the leaves, but in Cedrus and Larix only the leaves 

 drop off, new ones appearing on the old spur, as in Ginkgo. 



The leaves of conifers are small and always simple. Their arrangement 

 is spiral, except in the Cupressaceae, Avhere it is cyclic. The needle-like 

 type of leaf, as seen in Pinus and the other Abietaceae, is the dominant 

 one throughout the order (Fig. 291), but other types are also found. 

 Broad, flat leaves occur in Agathis and in many species of Podocarpus and 

 Araucaria. Scale-like leaves are characteristic of nearly all the Cupres- 

 saceae (Fig. 292). Where the adult foliage is scale-like, often the juvenile 

 leaves, appearing on seedlings, are needle-like. The flat leaves of conifers 

 have several or many parallel veins, while the needle-like and scale-like 

 leaves have but a single vein. 



A pine leaf is adapted to endure severe environmental conditions (Fig. 

 293). On the outside is a single-layered epidermis with heavily cutinized 

 cell walls and deeply sunken stomata. Beneath the epidermis are one or 

 more hypodermal layers also with thick walls. The mesophyll is compact 

 and peculiar in that the cells have infolded walls. It generally contains a 

 number of resin ducts. The central tissue of the leaf, enclosed by an 

 endodermis, contains one or two vascular bundles, the number depending 

 on the species. The xylem and phloem, nearly equal in amount and 



' These diameters are measured at a height of about 3 m. above the greatly swollen 

 base. 



