618 



APPENDIX A 



fernlike leaves bearing well-developed ovules at their tips or along the midribs; 

 the ova were probably fertilized by swimming sperms. They had all the appear- 

 ance of ferns and were originally classified as such, but discovery of their seeds 

 revealed their true position. 



The Cycads (*Bennettitales, fossil; Cycadales, recent) were important during 

 the Mesozoic era but are represented today by only nine genera and about 90 

 species. The plants are like palms or tree ferns in appearance (Figs. 26.14 and 

 A. 19), with a thick columnar stem that rarely branches and is often covered by 

 an armor of dead leaf bases, and with a crown of thick, feathery, stiff leaves. The 

 cycads are dioecious (as were their probable ancestors, the seed ferns), producing 

 male and female cones (Figs. 25.2 and A. 19). An interesting feature of the cycads, 



Fig. A.19. Reproductive structures of the cycad Zamia. A, habitus of plant. B, micro- 

 sporophyll bearing microspores (ms) . C, male cone with microsporophylls. D, section of 

 megasporophyll showing ovules (ov). E, female cone bearing megasporophylls. (Modified 

 from Turtox chart, courtesy General Biological Supply House, Inc.) 



shared with the ginkgoes and doubtless with the seed ferns, is the fact that the 

 sperms developed in the pollen tube are ciliated and able to swim, like those of 

 the bryophytes, lycopods and ferns. In those lower groups motile sperms are 

 necessitated by the method of fertilization; but in plants that have a pollen tube 

 the development of ciliated sperms is unnecessary and can only represent a sur- 

 vival of ancestral characteristics. Motile sperms, and the leaflike structure of the 

 cone scales of some species indicate that the cycads are the most primitive of 

 existing seed plants. 



Cycads are characteristically very slow-growing and long-lived plants. Indi- 

 viduals of some species certainly reach an age upwards of a thousand years, and 

 probably much more. Modern representatives of the order are practically confined 

 to tropical and subtropical regions, although some of the fossil forms were much 

 more widely distributed during the Mesozoic. Four native species of the genus 



