The work of the leaf is also interfered with by diseases which attack the leaves and cause parts or the whole leaf to turn yellow or brown or become blistered or filled with holes. The common remedy for most of these diseases is called the "Bordeaux Mixture." It is prepared as follows: Dissolve four pounds of blue vitriol (blue stone, or copper sulphate) in several gallons of water. Then slake four pounds of lime. Mix the two and add enough water to make a barrelful. The mixture is then sprayed on the plants. For more detailed directions for spraying plants and combating insects and diseases write to your State Experiment Station and to the United States Department of Agriculture at Washington, D.C. [Illustration: FIG. 65. To show the giving off of gas by leaves, and that sunlight is necessary for it. The jars contain seaweed. _A_ was set in the sun and developed enough gas to float part of the plant. _B_ was left in the darker part of the room and developed very little gas.] [Illustration: FIG. 66. Seedling radishes reaching for light.] [Illustration: FIG. 67. Elm leaves injured by the "imported elm-tree leaf beetle," a chewing insect.] The work of the leaves of house plants is often interfered with by not giving them sufficient sunlight. Garden and field plants are sometimes planted so thick that they crowd each other and shut the light and air from each other, or weeds are allowed to grow and do the same thing, the result being that the leaves cannot do good work and the plant becomes weak and sickly. Weeds are destroyed by pulling them up and exposing their roots to the sun. This should be done before the weeds blossom, to prevent them from producing fresh seeds for a new crop of weeds. Some weeds have fleshy roots--for example, dock, thistle--in which food is stored; these roots go deep in the ground, and when the upper part of the plant is cut or broken off the root sends up new shoots to take the place of the old. Some have underground stems in which food is stored for the same purpose. The surest way to get rid of such weeds, in fact, of all weeds, is to prevent their leaves from growing and making starch and digesting food for them. This is accomplished by constantly cutting off the young shoots as soon as they appear above the soil, or by growing some crop that will smother them. The constant effort to make new growth will soon exhaust the supply of stored food and the weed will die. CHAPTER XIV STEMS WHAT ARE STEMS FOR?