Text-book of structural and physiological botany . grains suggest a homology with themicrospores of Selaginella ; and the corpuscula or secondaryembryo-sacs with the central cell of the archegonia in Vas-cular Cryptogams. In the anatomical structure of the woodGymnosperms resemble Dicotyledons in all essential par-ticulars (see p. 356).] The class is divided into threeorders : — Order i. Cycade^. The Cycadcae (Pig. 453) are all tropical treeswith simple stems, resembling Palms in their habit and Ferns in the modeof development of their leaves. Their columnar stems attain a consider-able diamet
Text-book of structural and physiological botany . grains suggest a homology with themicrospores of Selaginella ; and the corpuscula or secondaryembryo-sacs with the central cell of the archegonia in Vas-cular Cryptogams. In the anatomical structure of the woodGymnosperms resemble Dicotyledons in all essential par-ticulars (see p. 356).] The class is divided into threeorders : — Order i. Cycade^. The Cycadcae (Pig. 453) are all tropical treeswith simple stems, resembling Palms in their habit and Ferns in the modeof development of their leaves. Their columnar stems attain a consider-able diameter, are tuberous or globular when young, and often evenat a later period, and bear two kinds of leaves ; the one dry brown,hairy, sessile scales of a coriaceous texture and comparatively smallsize, the other large, stalked, simply pinnate foliage-leaves. They are 334 Stritctiwal and Physio log teal Botany, Glioecious. Both kinds of flowers are always placed in terminal conesat the apex of the stem, in the centre of the crown of leaves, and have. Fig. ^S3—Cycas circinalis, a Cycad (greatly reduced). no floral envelope or perianth, but consist simply of an axis, which isin the one case densely covered with stamens, in the other case bears special Morphology and Classification, 335 I.
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