272 PHYLUM XII. CYCADOPHYTA 



reduced to a simple tube, which contains usually two 

 large, multiciliated sperms (suggesting a correlation 

 between size and the number of sperms). In both 

 phyla, again, the megaspores develop from a spore 

 mother-cell (archespore) as tetrads, but while in the 

 Lycopods all four may become mature, 

 in the Cycads only one matures. In Ly- 

 copods the megaspores separate from the 

 sporangial tissue as they develop, and 

 normally are set free, while in Cycads 

 Yia. r46.— Cyoad ^^® smglc mcgasporc remains perma- 

 fnd sSms!^^' ^^^^ nently connected with and surrounded 

 by the sporangial tissue. So the embryo 

 sporophyte of the former normally develops outside of 

 the megasporangium^ while in the latter it does so in- 

 side of the megasporangium, and thus forms the seed. 



487. The lowest Cycads, the so-called **Seed-ferns" 

 (Class Pteridospermeae) , were abundant in the Paleo- 

 zoic period and are now known only from their fossil frag- 

 ments. They were long thought to be 



ferns of an ancient type, but are now 



known to have been seed-bearing plants. 



Apparently they were derived from the 



Marattias among the Old Ferns. Their 



leaves were fern-like in shape and struc- fig. 147.— Pterido- 



ture. Their stems were capable of in- and seTd. '''°'°''^'^'' 



creasing in diameter. It is now thought 



that the Seed-ferns constituted a group of vast extent in 



Paleozoic times. 



488. In the Common Cycads of the present (Class 

 Cycadineae) the sporophytcs are usually erect, woody, 

 little-branched trees, rooted below, and bearing terminal 

 crowns of evergreen, pinnate leaves. The collateral 

 vascular bundles are arranged cylindrically in the stem, 



