DEVELOPMENT OF PLANTS 321 



phyte. The seed plants are divided into two subdivisions based 

 upon the relation of the megasporangia to the sporophylls : 1. 

 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. 



Subdivision 1. Gymnospermae. Plants with Naked Seeds 



119. Origin of the Gymnospermae. — This group is principally 

 represented today 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 

 wonderfully well preserved and show that they were undoubtedly 

 related to the ferns. The modern group of gymnosperms have 

 been derived from these ancient forms and but a remnant have 

 survived the changes occurring upon the earth and the competi- 

 tion with the more highly specialized Angiospermae. A few 

 genera, however, are well represented today 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 two of the more important orders : The 

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



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

 tropical, though two 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 showing 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 

 decidedly palm-like in appearance, with stems 12 to 60 feet high. 

 The vascular bundles are collateral and arranged around a larse 



