658 A TEXTBOOK OF THEORETICAL BOTANY 



pycnoxylic, while in the latter case it is manoxylic, that is to say, loose, 

 soft and scanty, as in many of the Pteridosperms. 



The flowers take the form of cones made up of cone scales. In the 

 Conifers these are relatively small except in a few cases, but in the Cycads 

 the cones are very large, often a foot or more in length. These cones are 

 made up of an aggregation of sporophylls bearing sporangia in which the 

 spores are produced. The cones are generally unisexual, the male being 

 borne separately from the female, and frequently differing from it con- 

 siderably in shape and size. Hermaphrodite cones sometimes occur as 

 abnormalities. The female cones are long-lived structures, sometimes 

 remaining on the plants for several years before they become mature and 

 the seeds are ripened. The male cones are usually smaller and short-lived. 



The gametophyte generation is even more reduced than it is in any of 

 the Pteridophyta. Neither the male nor the female gametophyte ever possesses 

 an independent existence, and the male prothallus is represented only by one 

 or two cells. 



The Gymnospermae are divided into a number of orders, some of which 

 are represented only by fossil types. Others possess both fossil and living 

 forms, but the general indications are that the highest development of the 

 Gymnosperms was during the Jurassic period, and that the present-day 

 forms are the remains of a more numerous fossil group. 



The following are the orders generally recognized : — 



1. Cordaitales, e.g., Cordaites (fossil). 



2. Coniferales, ^.^., P///Z/5. 



3. Taxales, e.g., Taxus. 



4. Cycadales, e.g., Cycas. 



5. Bennettitales, e.g., Cycadeoidea (fossil). 



6. Ginkgoales, e.g.. Ginkgo. 



7. Gnetales, e.g.. Ephedra, Giietum, Welwitschia. 



The type genus, which will be described in detail, is given in each case, 

 except for Cordaitales and Bennettitales, which will be dealt with in 

 Volume III. 



The order of the groups given above is that in which they will be treated 

 in this book, which is dictated by considerations of convenience of description 

 and teaching. It does not represent a phylogenetic sequence, since in this 

 respect the Cycadales and Ginkgoales would rank as primitive types among 

 the living Gymnosperms. 



Coniferales 



This is much the largest order of the Gymnospermae. Its members are 

 nearly all evergreen trees or shrubs and include the giant Redwoods of 

 California {Sequoia), the largest trees in the world. Representatives of the 

 group are to be found in every country, but they reach their greatest develop- 

 ment in the northern hemisphere, especially in the colder zones of Canada, 

 Scandinavia, Russia and Siberia, where the Pines and Firs reach to the 



