Water Molds (Saprolegniacere) which arc minute filamentous, colorless plants living in the water on living and dead plants and ani mals. (Fig. 4). Each plant is a more or less branched thread, some portion of which pene trates the host and thus obtains food, while the other part is external and bears the repro ductive organs. The filaments are cylindrical, and are peculiar in having no cross partitions in the vegetative portions. They are to be re garded as composed of many cells which have not separated themselves by partitions. The nuclei are numerous, and very small.
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 re mains naked (that is, no cell wall is formed around it), and soon escapes by a rupture 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 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 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 oogones, or in plainer words they are egg-organs, and in them one or more eggs are produced. The second seg ments (slender) are male organs called an therids, and the protoplasm they contain has the function of the spermatozoids of many plants (and animals). At the proper time the an therids puncture the egg-organs, and by the in flow 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 (Peronosporacee) are much like the water molds, but instead of be ing aquatic, they live in the tissues of land plants. Like the water molds they are com
posed 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 con ditions. In a few species, however, including the organism of the Late Blight of the potato (Phytophthora infcstans), 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 nal part of the fungus short branches grow out into the air, and these become swollen ter minally 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 struc tures 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 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 differ ences being quite immaterial for the present discussion. (Fig. 7).
It is evident from a comparison of the struc ture 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 (Vaucheriucece) by the adop tion 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 dif ference in the environment of the plants.
Black Molds (Mucoracez) show an addi tional 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 vege table, 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 suffi cient moisture. Organic substances which are dry are not attacked by black molds.