Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/147

Rh VEGETABLE KINGDOM 131 CLASS I. GYMNOSPERM^:. The carpels are sometimes absent, and, when present, they do not form an ovary; hence the ovules are naked. There is no stigma, conse quently the pollen-grain comes into direct contact with the ovule. The whole of the female prothallium is formed in the embryo sac before fertilization. The female organ is in most cases a fully developed archegonium. The embryo is developed from a portion only of the fertilized female cell or oospore (meroUastic embryogeny] ; frequently several embryos are developed from one oospore (polyembryony}. The flowers are dicecious or diclinous. This class consists of the following orders : Order 1. Cycadaceie : characterized generally by the unbranched stem and by the large branched leaves. The order includes nine genera: Cycas, Dioon, Ccratozamia, Zamia, Macrozamia, Enccphal- artos, Stangeria, Bowenia, and Microcycas. The genus Cycas is peculiar, in that the carpels are not borne on special shoots, so as to form flowers, but on the main shoot, in the same way as the foliage leaves. The stamens of Cycas, as also both the stamens and carpels in the other genera, are highly modified sporophylls which are borne on special shoots forming cone-like flowers. Order 2. Coniferas : characterized by the much-branched stem and by the very numerous, small, unbranched leaves. The staminal flowers are generally cone-like. This order includes the Pines, Firs, Larches, Yews, Cypresses, &c., arranged in the following families : Series A. Pinoideae : the carpellary flowers are cones : Fam. 1. Abictincss. Fam. 3. Oupressinese. ,, 2. Araucaricx. ,, 4. Taxodinese. Series 15. Taxoidese : the carpellary flowers are not cones : Fam. 1. Taxinese. Fam. 2. Podocarpese. OrderS. Gnetacese: habit various ; the flowers are not cones, and have a perianth. This order includes the three genera Ephedra, Giictum, and Welwitschia. In Gnetum and Welwitschia the female organ (archegonium) is reduced to a single cell, the oospherc. Mutual Affinities and Phylogeny of Gymnospermse. There is but little resemblance between the Cycadacese, and the other two orders. Gingko or Salisburia, a remarkable genus which is usually placed in the Taxinese, though it is in many respects peculiar, resembles the Cycadacess in the structure of the arche gonium, in the fleshy character assumed by the outer portion of the seed-coat, and in the fact that the development of the embryo does not begin until after the seed has been shed. Probably the fossil plants known as the Cordaitaccse may be regarded as a group inter mediate between the Cycadacese, and the Conifers,, The Coniferse and the Gnetaceaz are more closely allied, so much so in fact that the latter have been regarded by various writers as a family of the Coniferse. The structure of the flowers in Ephedra is very similar to that in Taxus. The connexion of the Gymnosperms with the Ftcridophyta is un doubted. The development of the microsporangium in the Gymno sperms is the same as that of the sporangium of Lycopodium. The development of the macrosporangium presents in most cases the characteristic Phanerogamic features ; but in certain cases (Ccratozamia longifolia, Cuprcssus scmpervirens, Callitris quadri- valvis) there is a mass of sporogenous cells, as in the sporangia of the Pteridophyta, though only one of these cells is fertile, and it produces a single macrospore without division. In fact, there is a marked general resemblance in the development of the macrospor angium between these species and Isoetes. Another point of importance is the fact that, although the female prothallium (endosperm) is under ordinary conditions entirely and permanently enclosed in the macrospore (embryo sac) a feature which is charac teristic of the Phanerogamiayet in the Cycadacese, if the first- formed archegonia are not fertilized, the prothallium resumes its growth and bursts the macrospore, coming to the surface of the ovule and turning green in consequence of exposure to light. Tak ing into consideration their general habit and the peculiarities in the development of their sporangia, it appears that the Cycadacav find in the Eusporangiate Filices their nearest allies among the Pteridophyta. This suggestion presupposes the existence of hetero- sporous Eusporangiata, although all the known forms are homo- .sporous. But this difficulty will be met, if, as suggested above, it turns out that Isoetes really belongs to the Eusporangiate Filices. The Coniferse resemble more nearly the Lycopodinee, such as Lyco podium and Sclaginclla, in their general habit and in the cone-like form of their flowers. However, the position of the pollen-sacs on the under surface of the peltate sporophyll is a feature which connects them with the Equisetinse ; but, on the other hand, the ligular outgrowth on the upper surface of the maerosporophyll in Araucaria, which in the Abietinese. becomes the large ovuliferous scale, is another point of resemblance to Selaginclla and Isoetes. In view of these resemblances to the Lycopodinse and the Equisetinsz, it is hardly possible to trace back the Coniferse, to either of these groups : it is more probable that they sprang from forms of Pterido phyta intermediate in character between the Lycopodinss and the Equisctinse. The mutual relations and the phylogeny of the three genera of Gnetaccse cannot at present be determined with any de gree of probability. The one point which affords any clue is the resemblance in general structure between the flowers of Gnetum and those of the Taxinese. The flower of Welwitschia, more than that of any other Gymnosperm, resembles that of an Angiosperm, especially in the fact that the staminal flower contains a rudi mentary ovule. CLASS II. ANGIOSPERM^:. The carpel or carpels form an ovary, in which the ovules are enclosed. A portion of the carpel forms a stigma, which receives the pollen-grains. The pollen-grains germinate on the stigma, and therefore do not come into direct relation with the ovules. A part only of the female prothallium (primary endosperm} is formed before fertilization, the remainder (secondary endo sperm) after fertilization. The female organ is a reduced archegonium, consisting merely of the female cell (oosphere). The general occurrence of both macrosporophylls and micro- sporophylls in the same flower is a characteristic feature of this group, as is also the whoiied arrangement of the floral leaves. The Angiospcrmse are divided into two sub-classes, the Mono cotyledons and the Dicotyledons, according to the number of the primary leaves or cotyledons of the embryo ; hence, in the former sub-class the first leaves are alternate, in the latter opposite. In the Monocotyledons, with few exceptions, the growing-point of the embryonic stem is lateral ; in the Dicotyledons it is always terminal. There are other distinguishing features, such as the parallel vena tion of the leaves of Monocotyledons and the reticulate venation in Dicotyledons ; but they are not sufficiently constant to be of much taxonomic value. The Monocotyledons may be conveniently arranged in the following series : Series 1. Nudiftorse, : usually diclinous or dioecious ; perianth wanting or rudimentary. This series includes the Spadicifloree (Aroids, Palms, Naiadeae) and the Glumiflor& (Grasses, Sedges). Series 2. Pctaloidese : usually monoclinous ; perianth present, and usually well-developed and coloured. This series includes the Lilies, Orchids, Irises, &c. The Dicotyledons are usually classified as follows : Series 1. Monochlamydese : usually diclinous or dicecious ; perianth absent or simple. Series 2. Polypctalse: usually monoclinous; perianth usually con sists of calyx and coloured corolla, the petals being free. Series 3. Gamopetalse : usually monoclinous ; perianth usually consists of calyx and coloured corolla, the petals being coherent. Phylogeny of the Angiospcrmse. There can be no doubt that the Angiospcrmas have been derived from the Pteridophyta ; but it is a question whether they have had an independent origin from that group or whether they are to be traced back to it through the Gymnosperms. In view of the wide gulf which separates the Angiosperms from even the highest Pteri dophyta, and of the affinities of the Gymnosperms to the Ptcri dophyta on the one hand and to the Angiosperms on the other, the latter suggestion would appear to be the more probable. Although the Monocotyledons and the Dicotyledons have so many features in common, it is probable that they have not had the same origin. The Gymnospermous forms to which the Dicoty ledons are most nearly allied are the Coniferse, and Gnetum among the Gnetaccse. The marked Angiospermous characters of Gnetum, and its general similarity in habit to a Dicotyledonous plant, afford some ground f or regarding this genus as the starting-point of the Dicotyledons. But more probably the Dicotyledons are to be traced back to the Conifer as-, and the Gnctaccse to be regarded as a lateral offshoot of the Coniferse, with an Angiospermous tendency, but not leading on to higher forms. The Gymnosperms which most re semble the Monocotyledons are the Cycadaceie ; and, if it be assumed that the Angiosperms have sprung from the Gymnosperms, it is to this order of the latter class that the Monocotyledons are to be traced. The Monocotyledons in which this resemblance is most conspicuous are those, such as the Aroids and Palms, which belong to the series Nudifiorie. Literature. On the PJtanerogamia, see Goebel, Outlines of Classification and Special Morphology, Oxford, 1887 ; Le Manut and Decaisne, Traitt, de Kotaniqve (English eil. by iiooker, 180S) ; and Engler and Prautl, Die Pflanzenfamilien, Leipsie (in course of publication). On the phylogeny of plants, see Nageli, Theorie d. Abxtammunrislehre, Munich, 1884 ; and Saporta and Marion, L f.volution du liegne Vi^lal (Int. Sci. Series, vols. xxxix. and lii.), Paris, 1881-85. Descriptions of the numerous natural orders will be found in the various text-books which treat of systematic botany, as well as in the above. (S. II. V.)