354 PLANT MORPHOLOGY 



lies. (7) An arehegoniiim complex is present only in the Taxodiaceae and 

 Ciipressaceae. This is an advanced character. 



The oldest families of conifers are the Abietaceae and Araucariaceae. 

 Although it is uncertain which is the more ancient, much evidence from 

 the vascular anatomy of both living and extinct forms indicates that the 

 Abietaceae are the ancestral stock of conifers. The Araucariaceae seem 

 to have given rise to the Podocarpaceae and Taxaceae. The Taxodiaceae 

 and Cupressaceae are younger than the other families and have probably 

 sprung from the Abietaceae. 



7. Gnetales 



This is the highest order of gymnosperms. It includes 3 peculiar genera 

 of diverse habit and distribution. Ephedra, with about 35 species, inhab- 

 its arid parts of the Mediterranean region, tropical and temperate Asia, 

 and western North and South America. Welwitschia, with a single spe- 

 cies, is found only in arid parts of western South Africa. Gnetum, with 

 30 species, occurs in the tropics of South America, Asia, and Africa. The 

 fossil record of the Gnetales is very fragmentary and does not extend 

 beyond the Tertiary. 



Sporophyte. The species of Ephedra are low, much-branched shrubs, 

 seldom exceeding 2 m. in height, with long-jointed green stems bearing 

 opposite or whorled scale-like leaves (Fig. 302.4). Some of the species are 

 trailing. Welwitschia is a large turnip-shaped plant with a tuberous stem 

 about 1 m. in diameter and about one-third as tall (Fig. 303). It bears a 

 single pair of terminal, elongated, strap-shaped leaves with parallel vena- 

 tion. They trail along the ground, reaching a length of 3 m. or more and 

 becoming split into numerous segments. Except for the cotyledons, 

 these are the only leaves the plant ever has. Most of the species of 

 Gnetum are woody vines, but a few are shrubs or small trees. They have 

 oval, leathery, opposite leaves that are net-veined and 5 to 8 cm. long 

 (Fig. 304A). All three genera are cyclic in the arrangement of their 

 leaves and sporophylls. 



The most distinctive feature of the vascular anatomy of all three genera 

 is the occurrence of true vessels (tracheae) in the secondary wood. These 

 are present in addition to tracheids. Resin canals are absent. The 

 endarch condition prevails throughout the plant body, all traces of 

 mesarch structure apparently having been eliminated. 



The strobili of Ephedra and Gnetum are usually monosporangiate and, 

 as a rule, the two kinds occur on separate plants. The strobili of Wel- 

 witschia are also functionally monosporangiate but, in the staminate 

 strobilus, each set of stamens surrounds an abortive ovule, thus indicating 

 an ancestral bisporangiate condition (Fig. 305C). In all three genera 



