66 AN INTRODUCTION TO THE STUDY OF FOSSILS 



various cosmopolitan leaf genera of the Mesozoic, as Podozamites, 

 Zamites, etc., are hypothetically attached. 



Family 2, CycadecE, the existing cycads or sago palms. — 

 Plants with thick, columnar stems, at times attaining a height 

 of thirty to sixty feet. The trunks are covered with an armor 

 of old leaf-bases and bear a crown of large, usually once pinnate 

 leaves, with from one or two apical cones to as many as thirty 

 in one Australian form. The fructifications are in nearly all 

 genera cones, either male or female, the sexes being separate on 

 different plants. In the female plant of the living Cycas, how- 

 ever, the reproductive organs are not compacted into a cone, 

 but consist of simple, leaf-like blades on which are borne the 

 unprotected seeds. This is the simplest arrangement of repro- 

 ductive organs among living seed plants and is reminiscent of 

 ferns. 



Another primitive character of the cycads is their method of 

 fertilization. In most of the living seed plants the male cells are 

 carried by the pollen tube to the ovule (Fig. 11, E'). In the 

 cycads and ginkgos alone among seed plants the male cells are 

 ciliated and motile, swimming actively to the ovule after the 

 rupture of the pollen tube, as is usually the case in the crypto- 

 gams, — the ferns, mosses and many algae. This motility of 

 the male cell is a survival of the earlier stages of the process of 

 plant evolution when water was an essential medium for the 

 process of fertilization, and of a still earlier stage when the whole 

 plant body was adapted to life in the water. 



Living cycads are tropical. There are nine genera, of which 

 Cycas, the best known eastern genus, is found in Asia and 

 Australia, and Zamia is the most conspicuous American 

 genus. They existed in the Mesozoic in some number, but so 

 far definite evidence of them remains meager ; it is the abundance 

 of their kindred, the Cycadeoideae, that marks out the Meso- 

 zoic as the " Age of Cycads," or as some name it " The Age of 

 Proangiosperms." The genus Cycas has been found in the 

 Lower Jurassic, though its leaf type goes back much farther. 



