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FO UR TH GROUP. — SEED-PLANTS. 



time as the embryo ; both sometimes remain in a rudimentary condition. The 

 pollen-grain also suffers division of its contents. Branching usually axillary and 

 from the axils of all the foliage-leaves. 



A. Monocotyledons. The first leaves of the embryo are alternate. Endo- 

 sperm usually large, embryo small \ 



B. Dicotyledons. The first leaves of the embryo are in a whorl of two 

 members. Endosperm often rudimentary, often consumed by the embryo before the 

 ripening of the seed. 



I. GYMNOSPERMAE. 



The class of Gymnosperms, composed of the orders Cycadeae, Coniferae, and 

 Gnetaceae, includes plants of strikingly different habit, but which are seen to be 

 connected with one another by their morphological characters, by peculiarities in the 

 formation of their tissues, and especially by their sexual reproduction. They occupy at 

 the same time an intermediate position between Vascular Cryptogams and Angio- 

 sperms, approaching most nearly to the Dicotyledons, especially in anatomical 

 structure. 



The pollen-grains shew their character as microspores by their division before 

 pollination into two or more cells, one or some of which form a very rudimentary 

 prothallium -, while another and that the largest cell developes the pollen-tube, when 

 the pollen-grain has reached the nucellus of the ovule. The pollen-sacs or 

 microsporangia are always outgrowths from the under side of formations, the stamens, 

 which are undoubtedly of the nature of leaves ; they are formed in larger or 

 smaller numbers or in pairs on each staminal leaf, and do not unite together as they 

 grow. 



The ovule, which is almost always orthotropous and usually provided with one 

 integument only, is either the metamorphosed extremity of the floral axis itself, or is 

 a lateral shoot from below its apex, or springs from the axil of a leaf or from 

 peculiar placental structures, or lastly is formed on the upper side or on the margins 

 of carpellary leaves. These leaves never cohere in the Gymnosperms into a true 

 ovary before fertilisation, but nevertheless often grow considerably during the 

 ripening of the seeds, and close together and conceal the seeds until they are ripe, 

 when they open again to let them fall out ; at the same time it is no rare thing for 

 the seeds to remain entirely naked from first to last. The embryo-sac is formed in 

 the small-celled nucellus from the originally hypodermal archesporium at some 

 distance beneath the apex and near the base, and continues till the time of fertilisation 

 to be surrounded by a thick layer of the tissue of the nucellus. The rudiments of 

 several embryo-sacs sometimes appear in a nucellus, but one only is fully developed. 



1 [In a considerable number of Monocotyledons, as in Dicotyledons, the small amount of 

 endosperm formed is consumed by the embryo before the ripening of the seed. 



^ See Strasburger, Neue Unters. ii. d. Befruchtungsvorg. b. d. Phanerog. He is disposed to 

 regard the whole pollen-grain as the homologue of an antheridium, and the formation of the small 

 cells (the prothallium of the text) as a physiological process, viz. the separation of substance not 

 essential in the sexual process. He holds the same view with regard to the microspores in the 

 Ligulatae (see page 284).] 



