QYMNOSPERMJS. 



165 



Strohis), is very valuable. Very many of the Cedars, 

 Pines, Firs, Arbor Vitse, Yews, are very ornamental. The 

 Big-tree of California {Sequoia gigantea), growing only on 

 the western slopes of the Sierra Nevada mountains, attains 

 a height of more than three hundred feet, and a diameter 

 of twenty to thirty feet. 



3. Gnetaceae. The Joint Firs are undershruhs, or 

 small trees, and, except the peculiar WelwiUehia, have 

 jointed, rush-like stems, 

 and opposite, setaceous, 

 or oval leaves. Unlike 

 the previous Gymnos- 

 perms the flowers have 

 a perianth, or floral 

 envelope. This may be 

 single and bifid, or com- 

 posed of two or more 

 bract-like bodies (phyl- 

 lomes). The order in- 

 cludes three genera, the most remarkable of which is the 

 Welvdtschia, which has but one species, growing in South 

 Africa. Its trunk is a foot or two in diameter, and arises 

 one foot above ground. There is a stout tap-root branch- 

 ing below. The top of the stem is flattened with a depres- 

 sion across its diameter. There are only two leaves arising 

 from grooves, near the top of the stem ; they seem to be 

 the persistent cotyledons, and grow to be six feet long. 

 Scarlet cones are produced on peduncles arising from the 

 axis of the leaves. 



Fig. 280. Transverse section of a Pine leaf, highly magnified, showing the resitt 

 canal (r.c.) ; j/, stomate; *>■, hypoderm ; cA/, chlorophyll ; a, air-cavity. 



