THE SPERM ATOI'IIYTA 349 



To describe adequately the various orders of which this huge 

 division is composed is quite impossible within the limits of our 

 text. Aside from its numerous and varied living members it also 

 includes a great number of fossil types, many of which are impor- 

 tant in reconstructing for us the steps in the evolutionary history 

 of the seed plants. We shall attempt to present the salient 

 characters of the more important groups only, and to indicate 

 their probable relationship to each other and their place in the 

 phylogeny of the whole series. 



Gymnospermae or Gymnosperms. — Two classes of seed plants 

 are commonly recognized, the gymnosperms and the angiosperms, 

 differing chiefly as to the manner in which the ovules are borne. 

 The gymnosperms are the most ancient of seed plants, and a 

 varied and heterogeneous group. All agree in possessing ovules 

 and seeds which are borne openly on the megasporophyll or 

 carpel, freely exposed to the air and to the direct contact of 

 pollen grains, and not inclosed in an ovary as they are among the 

 angiosperms. There is good reason to believe that the earliest 

 gymnosperms arose from fern-like ancestors, for several remark- 

 able fossil plants from the Coal Period have been discovered 

 which possessed typically fern-like foliage, and were long thought 

 to be true ferns, but which are now known to have borne 

 undoubted, though primitive, seeds. This group, sometimes 

 called the Cycadofilicales or cycad-ferns, has long been extinct, 

 nor have any of its near relatives survived. The most primitive 

 living gymnosperms are the Cycads. 



1. Cycadales or Cycads. — Cycad stems are typically stout and 

 unbranched, and bear at the top a crown of large, pinnate leaves 

 (Fig. 219). A few species have tall, columnar trunks and thus 

 superficially resemble tree ferns or palms. Internally, the stem 

 possesses a large pith and cortex but its woody tissue is rather 

 weakly developed, although the fibro-vascular system is often 

 complicated by the occurrence of several concentric rings of 

 bundles instead of a single ring. 



The male and female sexual structures occur on separate plants, 

 and the sporophylls are borne in terminal cones (Fig. 220). In 

 the genus Cycas, the ovulate sporophylls ("female" cone scales) 

 are large and lobed, showing a slight resemblance to foliage 

 leaves, and bear ovules along their edges (Fig. 221). In the other 

 members of the order the cones are more compact and the sporo- 

 phylls or cone scales have lost their leaf-like character. Each 



