carbohydrates. Accordingly plants which are not green, and which as a consequence do not make carbohydrates, often grow in darkness or in feeble light. This is quite characteristic of the fungi, great numbers of which grow as well in darkness as in light, or in some cases grow even much better in the darkness than they do in the light.

The fungi are very numerous, some recent estimates placing the number of species as high as 250,000, of which not more than one-third have as yet been described. They occur wherever there is organic matter of any kind upon which they can subsist. Wherever there are living plants or animals there are fungi which obtain food either from the living cells of their hosts, or the dead and cast-off cells and tissues. Some species occur in the lower layers of the air, in all exposed waters and in the soil. They are the most numerous of living things when we consider individuals alone. They range in size from extremely small to many centimeters in length. The smallest are far too minute to be seen by the naked eye, some being visible only by the aid of the most powerful microscopes. Of some of the smallest species it would require 25,000 to 30,000 placed side by side to measure one inch. On the other hand there are toadstools a foot or so in height and diameter, and puff-balls two to three feet in diameter have been recorded.

The fungi as thus described are found in four of the grand divisions (phyla) of the vegetable kingdom.

Phylum, Myxophyceæ. The Slime Algæ. Here are gathered a thousand or so species of microscopic aquatic plants in which the cells have a very low organization. No distinct nucleus is present, and the coloring matter in the typical plants pervades the whole cell, and is of a bluish or brownish-green color instead of a bright green as in higher plants. They reproduce by simple fission, and by the production of spores. There is no hint of any sexual reproductive process. They occur in ponds, pools and streams, to which they give a greenish color by their great numbers. In decaying they usually give off a fetid odor.

While the typical Slime Algæ are greenish — and are known as green slimes-many have become parasitic or saprophytic, and have lost their green color. These colorless species are known as Bacteria, and are the lowest of the fungi.

Bacteria (Fig. 1) are then to be regarded as colorless green slimes, their lack of color

coccus, Staphylococcus, Sarcina, etc. Other bacteria consist of cylindrical cells which tend to adhere end to end in filaments or rods. In the genera Bacillus and Bacterium the rods are straight or little curved, and short or of moderate length, while in Vibrio and Spirillum the rods are more or less spirally curved. In still other genera, as Crenothrix, Leptothrix, etc., the rods are elongated. The study of Bacteria

FIG. 2.-A, Olpidium brassica, one of the simplest of the fungi (Synchytriaceae), parasitic in cells of a crucifer: B, three zoospores. Highly magnified.

in relation to diseases of man and other animals and of plants, and to soils, etc., has developed into the science of Bacteriology (q.v.). Many botanists now, on that account, do not include Bacteria among the Fungi. See BAC

TERIA.

Phylum Chlorophyceæ and Phylum Siphonophyceæ. These plants, of which there are probably nearly 2,500 species, may very properly be called sea-weeds, since they are typically aquatic, living in the salt and fresh waters of the earth. Typically they are bright green, and the cells of which they are composed have well-formed nuclei. However, the chlorophyll is confined to definite portions of the protoplasm, and is not diffused throughout the cell. Some of the lower species are spherical, rounded cells, but for the most part they consist of filaments of cylindrical cells, or in some instances they are masses of cells constituting leafy-stemmed plants. They reproduce by fission as in the Slime Algæ, but in addition all, or nearly all, Slime Algæ reproduce sexually also. In the simplest cases of sexuality, two equal and similar cells detached from older plants fuse into a new and larger cell, and then this new cell grows into a new plant. Sometimes the new cell becomes covered with a thick wall, and for a time ceases activity as a "resting spore," before it develops into a new plant.

While most of the plants of these two phyla are green plants, several hundred species have become parasitic or saprophytic in habit (Figs. 2 to 10) and have therefore lost their color, and become fungi. Among these are the following families, namely:

Gall Fungi (Synchytriacea) are beyond reasonable doubt to be regarded as hysterophytic forms of the one-celled class (class Protococcoidea) of the first of these two phyla. These fungi consist of single cells which enter the cells of their living host plants and there enlarge, feeding on the host cells and causing an irritation of the tissues which often causes swellings which may be minute or in some cases very large (e.g., wart disease of potato tubers caused by Synchytrium endobioticum). The fungus cell eventually breaks up internally into numerous minute zoospores which escape

in various ways and infect new host cells. (Fig. 2). The family Chytridiaceae (Fig. 3) which are mostly parasitic in aquatic alga and fungi are probably closely related to these or to the next forms.

Water Molds (Saprolegniacea) which are minute filamentous, colorless plants living in the water on living and dead plants and animals. (Fig. 4). Each plant is a more or less branched thread, some portion of which penetrates the host and thus obtains food, while the other part is external and bears the reproductive organs. The filaments are cylindrical, and are peculiar in having no cross partitions in the vegetative portions. They are to be regarded as composed of many cells which have

FIG. 3. Rhizidiomyces apophysatus, another of the lower fungi (Chytridiacea); the root-like organs are parasitic in a cell of a water mold. Highly magnified. FIG. 4. Sa prolegnia thureti; A, fly with reproductive filaments; B, end of a filament forming zoospores. C, zoospores escaping. A, natural size; B and C, highly magnified.

not separated themselves by partitions. nuclei are numerous, and very small.

The

The more common mode of reproduction is as follows: A terminal portion of a branch forms a partition at some distance from the extremity and the protoplasm in this segment becomes denser, and a little later divides into a great number of small cells, each of which remains naked (that is, no cell wall is formed around it), and soon escapes by a rupture of the end of the segment. (Fig. 4). These escaped cells are known as zoospores, since they have a very active swimming motion, very like that of some of the lower microscopic animals. The similarity to the lower animals is shown still more by the identity in their locomotive organs, which consists of one or two slender

protoplasmic whips (flagella) by whose rapid lashing the zoospores are propelled. After a short period of activity they come to rest, when they cover themselves with a cell wall, and begin to elongate into a filament like that of the

FIG. 5.- Achlya racemosa, showing antherids and oögones. Highly magnified.

plant from which they came. Reproduction by means of zoospores is very rapid, since they are formed in such great numbers when conditions are favorable.

The sexual organs, which are rather rarely formed, consist first of an enlarged and rounded end segment in which the protoplasm is quite dense. From the sides of the branch below the end segment (or from elsewhere on the body of the plant) slender branches grow up and in turn their ends become cut off by cross partitions. (Fig. 5). The first end segments (the rounded ones) are oögones, or in plainer words they are egg-organs, and in them one or more eggs are produced. The second segments (slender) are male organs called antherids, and the protoplasm they contain has the function of the spermatozoids of many plants (and animals). At the proper time the antherids puncture the egg-organs, and by the inflow of the contents of the former the eggs are fertilized. Later these eggs may germinate and produce new plants like those on which they were borne.

Downy Mildews (Peronosporacea) are much like the water molds, but instead of being aquatic, they live in the tissues of land plants. Like the water molds they are composed of branching, non-septate filaments. The main body of the plant usually grows in the intercellular spaces of the host, where there is nearly as much moisture as under aquatic conditions. In a few species, however, including the organism of the Late Blight of the potato (Phytophthora infestans), the filaments grow directly through the cells of the host, killing them in advance by a poisonous secretion sent out by the fungus. (Fig. 6). From this inter

FIG. 6. Portion of a filament of Albugo candida, with its haustoria penetrating host cells. Highly magnified.

nal part of the fungus short branches grow out into the air, and these become swollen terminally into rounded segments, which are in fact short zoosporangia. Instead of forming zoospores at once, they first fall off and then

those that fall into water develop zoospores, much as in the water molds. As these structures are very minute, a droplet on a leaf is large enough for the germination of hundreds of the detached zoosporangia. Here again, the zoospores, after coming to rest, develop into new plants, which at once penetrate the host.. In some species the zoosporangia grow at once into a filament, without forming zoospores.

The sexual organs of downy mildews are much like those of water molds, the differences being quite immaterial for the present discussion. (Fig. 7).

It is evident from a comparison of the structure and reproductive organs of water molds and downy mildews, that the latter are derived from the former. Just as the water molds have been derived from the green plants of the Green Felt family (Vaucheriacea) by the adoption of parasitic and saprophytic habits, so by the change from strictly aquatic conditions to those found in the intercellular spaces of land plants, water molds have been changed to downy mildews. Every difference between the two families may be accounted for by this difference in the environment of the plants.

Black Molds (Mucoracea) show an additional modification of the water mold type. They are non-aquatic, mostly saprophytes, a few only being parasites. They live for the most part on dead organic matter, animal or vegetable, which is still moist, and but few species can live in the water. The commonest species live on the starchy and sugary substances in pantries, cellars and other places where these substances are found in the presence of sufficient moisture. Organic substances which are dry are not attacked by black molds.

Each black mold plant is a branching tubular filament, which has few cross partitions. One part of the plant usually grows in the substance of the organic matter, and another grows

FIG. 7.- Peronospora alsinearum, showing antherid penetrating the egg-cell. Highly magnified.

FIG. 8.- Mucor stolonifer. Highly magnified.

upward into the air. (Fig. 8). The former absorbs food matter, while the latter bears reproductive organs, as in the water molds. The ends of the aerial branches enlarge as in the two preceding families, but instead of forming zoospores, the protoplasm in the terminal segments forms many small spores, each covered with a cell wall. These spores are the homologues of the zoospores in previous families, but as the plants are not aquatic, these zoo

FIG. 9.- Mucor fusiger; A, young sexual organs; B, after fertilization. Highly magnified.

favorable conditions they quickly give rise to a new crop of mold plants. On the filaments which penetrate the nutrient substance, or grow over its surface, are to be found (rarely, however, in the common species) sexual organs somewhat resembling those of the two preceding families. (Fig. 9).

Insect Fungi (Entomophthoracea) are somewhat similar to black molds, but are parasitic in the body tissues of insects, and accordingly show considerable structural modifications. (Fig. 10).

Phylum Carpomyceteæ. The Higher Fungi.- This immense phylum, containing over 60,000 recognized species and probably twice as many, if not more, not yet studied, seems to have arisen from chlorophyll-bearing plants of the phylum Rhodophyceæ, the Red Seaweeds. These are mostly marine, although many fresh-water species are known. They reproduce sexually by the union of a non-motile sperm with the elongation of the oogone, termed trichogyne. The male nucleus passes down this to the egg nucleus with which it unites. The fusion nucleus thus produced divides very freely and the daughter nuclei pass out into many branching threads whose terminal cells (carpospores) are the reproductive cells of the plant. The whole structure of oogone, branching threads and carpospores is a "spore-fruit" and may or may not be surrounded by a protective structure of vegetative cells. In the Higher Fungi we find similarly a spore-fruit arising as a result of the sexual process. The ultimate cells are of three kinds, depending upon which one of the three classes is under consideration. (See VEGETABLE KINGDOM and PLANTS, MORPHOLOGICAL EVOLUTION OF). These classes are the Ascomyceteæ, Basidiomyceteæ and Teliosporeæ. Beside these there is a large group of fungi, certainly belonging to this phylum but which in the lack of knowledge as to their sexual reproduction cannot with certainty be assigned to any one of these classes. These are the Imperfect Fungi. All the plants of this phylum possess branching threads with numerous cross walls (septa) and usually one or two nuclei to a cell. Asexually they produce by conidia, cells cut off from the ends of