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SPERMATOPHYTA 407 
gametophyte and of the megasporangium (Fig. 454), or of the 
megasporangium alone, is known as a seed. The seed is the 
characteristic structure of the division Spermatophyta. Usually 
the nucellus almost completely disappears during the growth 
of the seed. 
CLASS GYMNOSPERMAE 
Theclass Gymnospermae is the more primitive of the two classes 
of the Spermatophyta and is characterized by megasporangia 
which are not inclosed in 
ovaries. It includes four 
orders, three of which 
are treated below. 
ORDER CYCADALES 
The cycads are a small 
group of plants with 
either a columnar (Fig. 
455) or a tuberous stem 
which bears a crown of 
leathery, pinnately com- 
pound leaves. In the 
stem there is a relatively 
large pith surrounded by Fig. 456. Cross section of trunk of Cycas 
a band of woody tissue Leer linnit 

which increases in width 
by secondary thickening (Fig. 456). A slight development of 
secondary thickening is found in some ferns, but the cycads are 
the most primitive living plants that have a great development 
of secondary wood. 
Both the megasporangia (ovules) and the mucrosporangia are 
borne on modified leaves known, respectively, as megasporophylls 
(Fig. 457) and microsporophylls (Fig. 458). The microsporophylls 
are relatively small, bear large numbers of sporangia on their 
lower surfaces, and are aggregated into definite cones (Figs. 499, 
460) produced at the apex of the stem. In all the genera except 
