Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/146

Rh 130 VEGETABLE KINGDOM genera Selaginclla and Isoetes. This arrangement is not accepted here, for the reason that Sclaginella and Isoetes have nothing in common beyond the ligule, and the fact that they are both hetero- sporous vascular cryptogams. It is in fact a question whether or not Isoetes should be included in the Lycopodinie at all ; for it has few and large leaves, and the embryo sporophyte has no suspensor, but it has a primary root. This being the case, it is impossible to frame any definition of the group Lycopodlnse, which would apply to all the members, if Isoetes be included in it. In many features Isoetes resembles the Filidnse, and especially the Hydropterideie. It may be suggested here that Isoetes is a representative of the hetero- sporcus eusporangiate Filidnse. The only member of the Lyco- podinse which Isoetes at all resembles is Phylloglossum. Mutual Affinities and Phylogeny of Ptcridophyta. Beyond the characteristics which they possess in common, and which have caused them to be collected into one class, there are no special points of contact between the three sub-classes of the Pteri- dophyta. The classes have each its own well-marked habit ; and it therefore appears probable that they are to be regarded as equiva lent branches of the Pteridophyta, springing from a common origin, in each of which a differentiation of homosporous and heterosporous forms has taken place. The relation of the homosporous to the heterosporous forms in each class is a point of interest. There is little or nothing to be said on this subject as regards the Equisetinas. But in the Filicinse, it appears probable that the simpler hetero sporous forms have sprung from the simpler homosporous forms, that is, the Salviniaeess from the Hymenophyllacese ; and in the Li/copodinse that Selaginella has sprung from Lycopodium. As suming that Isoetes belongs to the Lycopodinse, its nearest ally would appear to be Phylloglossum ; but this point must remain undecided until the whole life-history, especially the embryogeny, of Phyllo glossum is known, and its true systematic position thereby fixed. Various attempts have been made to trace back the sporophyte of the Pteridophyta to that of the Muscincse, the points of contact being the Hymenophyllacese on the one hand and Anthoceros on the other ; but these attempts cannot be considered to have been suc cessful. The differences between the known forms of sporophyte in the two classes are too great to be explained away on any hypo thesis. The case is different with regard to the gametophyte ; there are, in fact, many points of resemblance between the prothallium of the Pteridophyta and the gametophyte of the Muscineie. As Goebel and Bower have pointed out, the prothallium of some species of Trichomancs is a branched filamentous structure, closely resembling Moss protonema ; and in most ferns, as also in Equisetum and Lycopodium, the prothallium is at first filamentous. The fila mentous prothallium gives rise to one or more flattened expansions, on which the sexual organs, or at least the archegonia, are developed (archegoniophores}. Thus there are indications in the gameto phyte of many Pteridophyta of that distinction of protonema and sexual shoot which is so characteristic of the gametophyte of the Muscinese. But this remarkable correspondence in the gameto phyte does not warrant the conclusion that the Pteridophyta have sprung from the Muscineie. It rather indicates that they are groups having a common origin in the Confervoid Chlorophyceae, but diverging from the first in the relation of the sporophyte to the gametophyte, so that, whereas in the Muscineaz the sporophyte remains attached to the gametophyte throughout its whole exist ence, in the Pteridophyta the sporophyte develops as an independ ent organism. There can be little doubt that in the primitive Pteridophyta the two generations resembled each other, and that, as evolution proceeded, they became more and more widely dissimilar, the gametophyte losing, the sporophyte gaining, in both morpho logical and histological differentiation. An indication of this is still afforded by the general resemblance of the sporophyte of Phyllo glossum to the gametophyte of Lycopodium cernuum as described by Treub, and of the sporophyte of Lycopodium Phlegmaria to its gametophyte (Treub). It will probably be found, when material is obtained for investigating it, that the sporophyte and the game tophyte of Phylloglossum are very similar. The phylogeny of the Pteridophyta, as suggested in the foregoing remarks, is indicated in the following table : (Chlorophycea?) Efjuisetinai Homosporous 1 Leptosporangiate Filicinse Homosporous ( // y menoph yl locex) Lycop (Homo (Phylloglossum) sporous) (Lycopodium) sporous) Sdaginella. Hetero- sporous. Euspnrangiate Filicinae Homosporous Hydro- pterideas (Hetero- (^o sporous). (Heterc Heterosporou.s (Isoetes T). Literature of Pteridophyta. Affinitiesaml phylogeny: Prantl, Untersucliungen zur Morphologic der Gefasskryptogamen, i.-ii., Leipsic, 1875-81 ; Goebel, Annale~ du Jardin Botanique de Buitenzorg, vii., 1887; Bower, &quot;On some Normal am if Botany, i., Leap. Akad., du Jardin Botanique de, Buitenzorg, iii.-iv., 1884-86; Id., Annals o_ 1888; Kny, &quot;DieEntwickelungderParkeriaceen,&quot; iiiNovaActad. k.^wjj. ^i/uu&amp;lt;., xxxvii., 1875. For general works, see Goebel, Outlines of Classification and Special Morphology, Oxford, 1887; Sadebeck, &quot;Die Gefasskryptogamen,&quot; in Schenk s Handbuch dcr Botanik, i., 1879. SUB-KINGDOM IV. PHANEROGAMIA. These plants are commonly known as &quot;flowering plants&quot;; but they are more correctly designated &quot;seed-bearing plants,&quot; or Spermaphyta, for the production of a seed is the one feature which distinguishes the members of this sub-kingdom from those of the other sub-kingdoms. There is a definite alternation of generations in the life- history of these plants, but it is obscured by the extreme reduction which the gametophyte has undergone (see REPRODUCTION). The sporophyte is the plant ; a sus pensor is formed in its embryogeny ; and it is hetero sporous. The sporophylls are aggregated on special shoots, frequently with other floral leaves (bracteoles, perianth leaves), to form flowers. The two kinds of sporangia are in all cases borne on distinct sporophylls. In the micro- sporangium there are numerous fertile sporogenous cells, each of which produces four microspores ; in the macro- sporangium there are generally but few sporogenous cells, of which only one is usually fertile, and this one produces a single macrospore without division. The microspores are set free from the sporangium producing them, whereas the macrospore is not. It is this last peculiarity which determines the formation of the seed. In the above account of the asexual reproductive organs of the Phancrogamia, the terms employed are those which are applied in the case of the heterosporous Pteridophyta ; the advantage gained by the use of these terms is that the true homologies of the repro ductive organs in the two sub-kingdoms are kept in view. The rela tion between these terms and those more commonly in use is as follows : stamen = leaf bearing microsporangia (microsporophyll) ; carpel = leaf bearing macrosporangia (maerosporophyll) ; pollen-sac = microsporangium ; anther = sorus of microsporangia ; ovule (nu- cellus)=macrosporangram ; pollen-grain = microspore ; embryo sac macrospore. Stamens and carpels are commonly borne by the same individual and in the same flower ; but in many cases they are confined to distinct individuals (dicecious) or to distinct flowers (diclinous) on the same individual. Inasmuch as these plants are heterosporous, the sexual generation is represented by male and female gametophytes. The male gameto phyte, formed by the germination of the microspore (pollen-grain), consists of two or sometimes three or four cells, one of which becomes the male organ (pollen tube=;antheridium of other sub- kingdoms), while the rest have nothing to do with reproduction, but constitute the rudimentary vegetative portion of the prothal lium. The female gametophyte, commonly known as the endo sperm ( = female prothallium), is formed in the interior of the macro- spore or embryo sac, and usually does not come at all into relation with the exterior. From certain cells of this endosperm one or more female organs are formed, which are equivalent to, and in some Phanerogams closely resemble, the archegonia of the Pteridophyta ; each of these female organs possesses, as its essential constituent, the female cell (oosphere, ovum). The process of fertilization (see REPRODUCTION) depends upon the pollen-grains being brought near to the ovules, so that, when the pollen-grains germinate, the pollen tubes may be able, in the course of their growth, to come into close relation with the female cells which the ovules contain. In many cases the transference of the pollen-grains is effected by the wind ; the plants in which this is the case are characterized by their inconspicuous flowers, and arc termed anemophilous. In other cases the transference is effected by means of insects ; the plants in which this is the case are termed entomophilous, and are characterized by their conspicuous flowers, in which the perianth leaves are highly developed and brightly coloured, to attract the visits of insects, an end which is further attained by the secretion of scented volatile oils and of sugary liquid (nectar). It is the conspicuous flowers of these ento mophilous Phanerogamia which have earned for the whole group the designation of &quot;flowering plants,&quot; which, however, is by no means generally applicable. The Phanerogamia are divided into two classes, the and the ANGIOSPERM^E.
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