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 

 the sin g le megaspore remains perma- 

 nen tly 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 Ptendo- 



ture. Their stems were capable of in- s a p d e s r d . 8porophyte 



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 sporophytes 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, 



