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 
Fic. 146—Cycaad the single megaspore remains perma- 
ed epee °° 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 PrERIDOSPERMEAE), 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 8 
Marattias among the Old Ferns. Their’ #’"A : 
leaves were fern-like in shape and struc- — yy¢. 147.—Pterido- 
ture. Their stems were capable of in- &Pf7m, sPorophyte 
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 
CYcCADINEAE) 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, 
