34.0 PLANT MORPHOLOGY 



Summary. Ginkgo hiJoha is the sole survivor of an order that was 

 widespread and abundant during the Mesozoic. It has retained the 

 primitive reproductive features of its ancestors, its advance being wholly 

 in vegetative characters. The stem is an endarch siphonostele, almost 

 all traces of mesarch structure having disappeared. The pith is not large. 

 Characters that the Ginkgoales have in common with the Cordaitales 

 include the branching habit, thick vascular cylinder, venation of the 

 leaves, structure of the stamens, ovule structure, and swimming sperms. 

 The distinctive features of the Ginkgoales include the form of the leaves 

 and the structure of the strobili. The ovulate strobili and megasporo- 

 phylls are greatly reduced, the microsporophylls less so. The Ginkgoales 

 may have been derived from the Cordaitales, but are themselves a blindly 

 ending line. 



6. Coniferales 



The Coniferales constitute the largest order of living gymnosperms, 

 including 6 families, 40 genera, and over 500 species. They are dis- 

 tributed throughout the North and South Temperate Zones, with only a 

 few representatives in the tropics, where they occur at high altitudes. 

 Conifers have been found as fossils as far back as the Permian, having 

 probably been derived from the Cordaitales (Fig. 258). During the 

 Mesozoic they became numerous and diversified into as many families 

 as are represented today. As a group, the Coniferales reached their 

 climax during the Lower Cretaceous. 



Families. In the following outline, all the genera occurring in North 

 America are named, together with several others of particular interest. 



1. Abietaceae. This is a family almost entirely confined to the North- 

 ern Hemisphere. It comprises 9 genera and about 230 species. The 

 largest genus is Pinus, with 90 species. Other important genera are 

 Cedrus, Larix, Picea, Tsuga, Pseudotsuga, and Abies. Of the foregoing, 

 all but Cedrus are represented in North America. 



2. Taxodiaceae. Here belong 9 genera and 16 species, nearly all grow- 

 ing in the Northern Hemisphere. Only Sequoia and Taxodium are found 

 in North America. Aletasequoia, formerly known only in the fossil state, 

 has recently been discovered growing in western China. 



3. Cupressaceae. This family includes 10 genera and approximately 

 120 species. It is represented in both hemispheres. The following 

 genera are found in North America: Libocedrus, Thuja, Cupressus, Cham- 

 aecyparis, and Juniperus. The largest genus, Juniperus, has 60 species. 



4. Araucariaceae. This family has only 2 genera. Both Agathis, with 

 20 species, and Araucaria, with 12 species, are of wide distribution in the 

 Southern Hemisphere. 



5. Podocarpaceae. This family belongs almost exclusively to the 



