DEVELOPMENT OF PLANTS 353 



into the mature sporophyte. The seed plants are divided into 

 two subdivisions based upon the relation of the megasporangia 

 to the sporophylls: I. Gymnospermae, with sporangia, hence 

 with the seed, on the surface of the sporophyll. 2. Angiospermae, 

 with sporangia, hence with the seed, enclosed by the sporophyll. 

 It should be stated that this association of these two subdivisions 

 is artificial, the Gymnosperms being more nearly related to the 

 Pteridophyta than to the Angiosperms. 



Subdivision i. Gymnospermae. Plants with Naked Seeds 



119. Origin of the Gymnospermae. This group is principally 

 represented to-day by the cone-bearing trees. In very ancient 

 geological times, several groups of primitive gymnosperms, now 

 extinct, flourished and formed an extensive and varied forest 

 vegetation. Fossil remains of these various groups have been 

 w r onderfully well preserved and show that they were undoubtedly 

 related to the Pteridophyta. The modern group of gymnosperms 

 have been derived from these ancient lines and but a remnant 

 have survived the changes occurring upon the earth and the com- 

 petition with the more highly specialized Angiospermae. A few 

 genera, how r ever, are well represented to-day and they are of 

 great commercial importance as lumber. The sporophylls of 

 the gymnosperms are arranged in a strobilus as in the Equise- 

 tales and Lycopodiales, but the microspores and megaspores are 

 not associated, being developed in separate strobili. Attention 

 will be directed to only tw T o of the more important orders: The 

 Cycadales or Cycads and the Finales or cone-bearing trees. 



120. Order a. Cycadales or Cycads. These plants are strictly 

 tropical, though tw r o genera are subtropical, Zamia or coontie 

 in Florida, and Cycas, often called the sago palm, in China and 

 Japan. Many of them are extensively cultivated in green-houses 

 owing to their peculiar growth and attractive foliage (Fig. 244). 

 The cycads include less than 100 species of what was in geolog- 

 ical times, a very extensive alliance. They are of special in- 

 terest as howing unmistakable fern characters and because they 

 are the most primitive of our extant seed plants. The stems 

 of these plants are rather tuberous, though certain species are 



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