484 



GYMNOSPERMS 



GYMNOTUS 



GymilOSpenilS ( Gr. gymnos, ' naked ; ' hence 

 the name Gymnosperms, ' naked-seeded ' plants ), 

 the lower or more primitive group of seed-plants 

 (Phanerogams, q.v. ), differ in many points from 

 the higher group, the Angiosperms. The chief 

 differences are summarised in the article Angio- 

 sperms (q.v.). Gymnosperms consist of the orders 

 Cycadacese (q.v.), Coniferte (q.v.), and Gnetaceae 

 ( see SEA-GRAPE ). Although these orders do not re- 

 semble one another externally, their morphological 

 characters and mode of sexual reproduction are very 

 similar. In structure their stems resemble those of 

 Dicotyledons ( q. v. ) ; the secondary wood is formed 

 in concentric rings from permanent cambium, con- 

 tains tracheides with bordered pits, but no true 

 vessels ; and secretory passages are present in most 

 stems, containing resin in conifers and gum in 

 cycads. It is, however, from their mode of sexual 

 reproduction that we are able most clearly to assign 

 their place among plants, as a connecting link 

 between the higher cryptogams and angiosperms. 



1, The hermaphrodite Fern Prothallus contrasted with male 

 (a) and female (6) Prothalli of Equisetum; 2, above are 

 corresponding reductions of the sexual prothalli in 3, Salvinia, 

 4, Isoetes, 5, Cycad and Conifer, and 6, many Angiosperms. 

 A, microstores or pollen-grains ; c, male pronucleus ; d, 

 spermatozoid ; e, male prothallus. B, niacrospores. 



In gymnosperms we first meet with an organ which 

 is morphologically, and at the same time physio- 

 logically, a Flower (q.v.). The flowers are uni- 

 sexual ; and the plants either monoecious or dioe- 

 cious; while hermaphroditism prevails among Angio- 

 sperms. The male flowers are stamens bearing 

 pollen-sacs, which develop free unicellular pollen- 

 grains ; those three sets of structures being respec- 

 tively the homologues of sporophylls, microspor- 

 angia, and microspores of cryptogams. Each 

 pollen-grain divides into a large reproductive cell 

 and one or more vegetative cells (the male pro- 

 thallus of higher cryptogams). Each cell has a 

 nucleus, and that of the reproductive cell, the male 

 promicleus, is the homologue of the spermatozoid of 

 cryptogams. When the stamens are matured, the 

 sacs open ; the grains are shaken out, and some 

 are borne by the wind to the surfaces of ripe ovules 

 (macrosporangia of cryptogams). When a pollen- 

 grain readies an ovule it begins to germinate, its 

 coat ruptures, the reproductive cell grows at the 



expense of the vegetative to form a pollen-tube 

 (antheridium of cryptogams) which ultimately 

 penetrates the nucellus of the ovule, and its pro- 

 nucleus fertilises the pronucleus of the oosphere 

 (see FERNS). This is a step in advance of the 

 higher cryptogams, for their microspores are shed 

 from the parent plant, and germinate only in a 

 substratum where they develop into prothalli-bear- 

 ing antheridia from which spermatozoids are event- 

 ually set free. The microspores of Salvinia natans, 

 a heterosporous fern, form the only exception, be- 

 cause they develop prothalli and antheridia within 

 the sporangium. Spermatozoids can fertilise only 

 with the help of water ; while pollen-grains of 

 gymnosperms are carried by the wind to the female 

 flowers. 



In gymnosperms, then, we have a very marked 

 transition in the process of fertilisation. Sper- 

 matozoids readily pass down the neck canals of 

 archegonia and so reach the oosphere, but they 

 would be unable to pierce the nucellus of gymno- 

 sperms ; hence the necessity of a slow-growing pollen- 

 tube in the latter. The female flower is a macro- 

 sporangium borne at the end of an axis or shoot, or 

 a carpellary leaf (sporophyll), with a macrospor- 

 angium in its axil, on its upper surface, or on its 

 margin. The ovule has never more than one coat ; 

 while in many angiosperms there are two. Further, 

 the carpellary leaves never unite to form an ovary 

 round the ovule, which, therefore, remains naked 

 (hence the name); in angiosperms the ovules are 

 always enclosed in ovaries. The ovule is filled 

 at first with a mass of tissue, the nucellus, in 

 which is afterwards developed the embryo-sac or 

 mother-cell ( macrospore of cryptogams ) ; this sac 

 forms within itself a prothallus (also called endo- 

 sperm of phanerogams) which develops at its 

 anterior end several archegonia (see FERNS, fig. 2). 

 The endosperm of gymnosperms is formed before, 

 that of angiosperms after fertilisation. Inside 

 each archegonium is an oosphere which, after 

 fertilisation of its pronucleus by the male pro- 

 nucleus, develops the embryo. Part only of the 

 oosphere forms the embryo, the rest forms a 

 kind of nutritive yolk, thus resembling the eggs 

 of many animals. This is the only example 

 of meroblastic segmentation of the ovum in the 

 vegetable kingdom (see EMBRYOLOGY). The em- 

 bryo lies straight in the prothallus, and never curved 

 as in many angiosperms. Concealment of alterna- 

 tion of generations thus takes place for the first 

 time in gymnosperms. In vascular cryptogams 

 there are two distinct sets of individuals viz. the 

 asexual ( sporophy te generation ) represented by the 

 fern-plant, and the sexual (oophyte generation) 

 represented by the minute fern-prothallus. The 

 sexual individuals of cryptogams, with the excep- 

 tion of the microspores of Salvinia, lead independ- 

 ent lives for a time ; but those of phanerogams 

 are parasitic on the parent plant; and as parasitism 

 leads to degeneration of parts, so we have the pro- 

 thalli in gymnosperms reduced, and still more 

 reduced in angiosperms. The evolution of plant- 

 forms has thus been a progressive increase of the 

 sporophyte generation at the expense of the 

 oophyte, and this is in harmony with the character- 

 istically anabolic nature of plants. See Goebel's 

 Morphology of Plants, Sach's Physiology of 

 Plants, and Vines's Physiology of Plants. 



Gymnotus, or ELECTRIC EEL (G. electricus), 

 the most powerful of the electric fishes, occurs 

 in the fresh, water of Brazil and Guiana. It is 

 type of a family Gymnotidse among the Physo- 

 stomatous bony fishes, but is the only known species 

 of its genus. There are no dorsal nor strictly 

 cavidal fins, but the anal fin extends on to the end 

 of the tail ; there are no scales, and the eyes 

 are very small. The fish attains a length of 6 



