Papers in the Biological Sciences
Date of this Version
8-15-1894
Abstract
The Vegetable Kingdom is in other words the plant world, or the aggregate of organisms called plants. It is co-ordinate with the animal kingdom, and these two include all forms of organic life on the earth. It is not possible to define precisely the line which separates the lower portions of these two kingdoms; we may therefore regard them as simply branches of one great group, inseparable below, but widely divergent above. In attempting to separate plants from animals the most we can do is to bring together those characters which separate the greater number of plants from animals, and then to associate with the plants thus set off such of the remaining organisms as appear to be more plant-like than animal.
For the most part plants are cells enclosed in walls of cellulose, or aggregates of such cells, all or part of which contain chlorophyll, by means of which they are able to appropriate carbon from inorganic matter (carbon dioxide). The organisms thus set off constitute the bulk of the vegetable kingdom, and characterize its principal divisions. With these typical plants we must associate many which have lost some of their strictly vegetable characteristics through parasitism or saprophytism. Thus the thousands of species of fungi, while destitute of chlorophyll and incapable of appropriating inorganic carbon, are plants nevertheless, and are to be associated with those to which they show some structural similarity.
There are now known and described about 175,000 species of plants on the globe, and recent estimates made by Professor Saccardo show that this is probably less than one-half of the total number. This vast assemblage of organisms requires classification in order that we may study them and communicate our results to others. Accordingly similar species have been gathered into genera, similar genera into families, similar families into orders, etc. Finally we have been able, from a study of these groups, to make generalizations as to their probable relationship, and thus to form a genetic system in which all plants are included in six great BRANCHES, further subdivided into fifteen CLASSES. This system may be graphically represented by the accompanying diagram.
In the synopsis of the Flora of Nebraska presented below, the sequence is from primitive or simple forms to those which are derived or more complex. It will be seen by a comparison of the synopsis, with the diagram given above, that many cases occur in which several groups have had a common origin from which they have diverged, their highest forms differing most widely. It follows therefore that in the synopsis we are compelled to return again and again to these common points of origin in order to follow out successively the diverging genetic lines.
But it must not be forgotten that the derived forms have suffered degradation, as is notoriously the case with the fungi and other parasitic or saprophytic plants. Here the degraded form is the derived one, and accordingly it must be considered after the primitive form, although the latter may be actually more complex. In the flowering plants we find many cases of progressive simplification in following genetic lines. Grasses and sedges, while in many ways simpler than lilies, are in fact much further from the primitive monocotyledonous type. The oaks and walnuts, the willows and cottonwoods, with flowers, apparently quite simple, must be given positions in a natural system much further removed from the primitive types than many plants with a much more complete floral structure. These principles will account for the unusual position assigned to some of the families in the synopsis.
A careful study of the flora of Nebraska shows that not only are all the great BRANCHES of the vegetable kingdom represented, but that of the fifteen CLASSES, fourteen are represented, and that of the fifty-four ORDERS, forty-three are represented, while of the 386 FAMILIES there are representatives of about one-half. On the other hand, of the 175,000 species of plants now known, probably little, if any, more than two per cent occur within our borders.
Branch I.—PROTOPHYTA.
Uni- or multicellular plants, single or aggregated into loose clusters, or mechanically united into filaments, often provided with a thick membrane or sheath; filaments or clusters often aggregated into a thallus; true nucleus absent; color generally aeruginous through various shades of red and yellow, but never chlorophyll-green; asexual reproduction typical, by cell-division; sexual reproduction wanting.
In some genera variously moclified cells, called spores, are present, and in others peculiarly colored cells, called heterocysts. Both sometimes occur in the same individual. Fresh water or marine algae floating freely, or attached to a substratum, or rarely terrestrial. Chiefly holophytes, but one family, the Bacteriaceae, typically hysterophytic.
In the more primitive forms propagation takes place by means of fission, the resulting cells continuing as independent plants. In others the resulting cells remain in loose clusters, attached by mutual compression or imbedded in a gelatinous matrix, or in the higher forms united into a multicellular filament.
It is probable that the Protophytes and the chlorophyll-bearing algae have developed in different directions from the same point. Chroococcus and Gloeocapsa on the one hand, and Protococcus and Palmella on the other, represent the most primitive of the respective groups. Nothing definite is known as to the relationship of these forms.
Branch II.—PHYCOPHYTA.
Uni- or multi-cellular plants; thallus generally filamentous, often stratose, simple, or ramose, chlorophyll-green, or colorless; reproduction sexual and asexual; asexual reproduction by means of fission, by zoospores, or by cysts; sexual reproduction by means of isogametes or heterogametes.
Mostly inhabitants of fresh or salt water. A few are terrestrial, however, and several small families are parasitic or saprophytic. One class, Phaeophyceae, is composed of marine algae exclusively, and of course finds no representation in the state.
In Protococcus and related genera the phycophytes represent probably the most primitive forms of plant life. On the other hand, however, this branch stands as the great central stock from one part of which the long series of parasites and saprophytes have sprung, and from another the red sea-weeds and stone-worts.
Branch III.—CARPOPHYTA.
Multicellular plants; plant-body, for the most part, a parenchymatous tissue-aggregate, with or without chlorophyll; vegetative cells typically unmodified, cylindrical, or hexagonal; reproduction sexual and asexual; asexual reproduction in the chlorophyll series chiefly by means of tetraspores, in the hysterophytic series by means of stylospores, chlamydospores, and conidia proper; sexual reproduction by means of carpogones and antherids, resulting in the formation of a sporocarp.
Chiefly marine holophytes, or terrestrial hysterophytes. Plant body an undifferentiated aggregate of parenchyma-cells, forming a tissue mass, except in the Perisporiaceae, Oharophyceae and the unicellular Saccharomycetes. Chlorophyll is absent in most of the orders. When present, it is often more or less masked by other substances, as the red and purple coloring matters of the Rhodophyceae and the lime incrustation of the Oharophyceae. Asexual reproduction is typical of but two classes, Ascomycetes and Rhodophyceae. In the former, it results by means of conidia, stylospores, and, more rarely, by chlamydospores; in the latter uniformly by means of tetraspores. The fertilization of the carpogone by the contents of the antherid, typically through the medium of a trichogyne, produces a so-called sporocarp, which is characteristic of the branch. In the Oharophyceae, however, the fertilization does not result in the formation of a sporocarp. In the hysterophytes, moreover, sexuality decreases with the distance from the point of derivation of the group until it finally disappears, but at the same time without a corresponding modification in the production of the sporocarp.
'l'he relationships of the carpophytes are varied, and their inter-relations somewhat obscure. Through the holophytic series they connect in a nearly straight line, the Phycophytes with the Bryophytes, notwithstanding the evident break at the beginning of the series. On the other hand the hysterophytic series, which ends blindly at the upper end, probably falls into two natural divisions, one of which, represented by the Ascomycetes and Basidiomycetes, has perhaps had its origin in or near the Peronosporaceae, while the other represented by the Laboulbeniaceae, etc., has its derivation and relationship still involved in great obscurity.
Class III.—COLEOCHAETEAE.
Small green plants growing attached to submerged stems and leaves; thallus composed of branched rows of cells more or less united laterally into a flat, irregular or circular disk. Reproduction by sexually produced carpospores and asexual swarm-spores (zoogonidia).
The terminal cell of a branch which is to produce a carpospore swells, and the upper portion elongates into a narrow tubular process (trichogyne) which opens at the top. At the same time antherids develop from certain cells as small flask-shaped outgrowths, usually three or four from a cell. Each antherid thus formed cuts off from the mother-cell by a transverse wall, and the contents form a single biciliate antherozoid, which escapes and finds its way to the female cell, probably through the trichogyne. After fertilization, the female cell forms a wall around itself inside the old cell-wall, and the whole becomes enveloped by a coating of cells which grows up from below, thus forming a sporocarp with a single carpospore.
The Coleochaeteae are related to the Oedogoniaceae on the one hand and to the Florideae on the other. It is possible also that the origin of the great groups of the higher fungi is to be found in some such group as this. The nature and significance of the process of the formation of the sporocarp of the higher fungi has been a fruitful cause of discussion, and it has been commonly thought of late that it had no relation to the carpospore of the Coleochaeteae, or to the oospore of the Phycophytes, but was rather homologous to the asexual spore-formation of the lower fungi. But the Laboulbeniaceae, which according to recent investigation exhibit asexual reproduction of the same type as the Coleochaeteae and Florideae, indicate that this view is erroneous and that the origin of the higher fungi is to be sought in about the same place as that of the last named groups.
There is but one order and family.
Comments
Lincoln, Neb., 1894