Bacteria and yeasts are very much smaller plants than molds. While an individual mold plant, growing as stated above like a very small vine, branches out and spreads over considerable surface, bacteria (that is, those forms in which the pulp packer is interested) are very short, single rods, which, when multiplied in size 500 times by the microscope, appear to be from 1/32 to ⅛ of an inch long. Yeasts, when multiplied in size 500 times, usually appear to be from two to five times as large as a pin head. The rod-shaped bacteria referred to above are mainly the lactic acid, and acetic acid bacteria, both of which produce fermentation in tomato products. The lactic acid germ is the same one which causes milk to sour, while the acetic acid germ is the one which produces the acetic acid of vinegar. There are many other kinds of bacteria in tomato pulp, many of which are small, round cells. However, the bulk of these probably come from the soil and are natural to the tomato, and are not counted when the number of bacteria per cubic centimeter are estimated. The rod-shaped types indicate fermentation. [Illustration: Fig. 5.—Rod-shaped bacteria from tomato pulp, common in bad catsups. (×500.) ] Bacteria reproduce themselves with amazing rapidity. It has been found out by watching them multiply under the microscope that under favorable conditions one germ reproduces itself every 30 minutes, and in some cases reproduction is known to have been even more rapid than that. The method of reproduction is by simple division. The rod-shaped bacterium is just like a match in shape, only many thousand times smaller, and in the space of about 30 minutes it will divide itself in the middle, thus changing itself into two shorter rods. Each of these rods grows in length, and each of them then divides, making four rods. In 30 minutes more the four rods change into eight, etc. It is the simplest method of reproduction there is.