Many people who grow house plants have trouble in keeping them well clothed with leaves, for instance, the geranium and the rubber plant.  The leaves are constantly turning yellow and dropping off or drying up.  This sometimes occurs from over-watering or not sufficiently watering the soil in the pot or box.  If the watering is all right the trouble may occur in this way:  The air of the house is quite dry, especially in winter.  As a result transpiration from the leaf may be excessive.  More water is transpired than is necessary, consequently more is pumped by the roots and with it more food is sent to the leaf than it can take care of.  As the excess of water is transpired the excess of food is left in the leaf.  The tendency is to clog its pores and therefore interfere with its work, and gradually weaken and finally kill it.  The remedy for this is to spray the leaves frequently so as to keep the air about them moist and so check transpiration.  Keeping a vessel of water near them helps also as this tends to keep the air moist.  Dust sometimes chokes the leaves.  Washing or spraying remedies this.

Sometimes house plants, and out-door plants as well, become covered with a small, green insect called the plant louse or aphis.  This insect has a sharp beak like a mosquito and it sucks the juices from the leaf and causes it to curl up, interfering with its work and finally killing it.  Frequent spraying with water will tend to keep these away.  A surer remedy against them is to spray the plants with weak tobacco water made by soaking tobacco or snuff in water, or to fumigate them with tobacco smoke.  Sometimes the under side of the leaf becomes infested with a very small mite called red spider because it spins a web.  These mites injure the leaf by sucking sap from it.  They can be kept in check by frequent spraying for they do not like water.  If, then, we are careful to frequently spray the leaves of our house plants we will have very little trouble from aphis, red spider or over transpiration.  The aphis, or plant louse, is often very numerous on out-door plants, for instance, the rose, chrysanthemum, cabbage, and fruit trees.  They vary in color from green to dark brown or black.  They are treated in the same way as those on the house plants.  Some familiar out-door insects which interfere with leaf work are the common potato bug, the green cabbage worm, the rose slug, the elm tree leaf beetle, the canker worm, the tomato worm.  These insects and many others eat the leaves (Fig. 67).  They chew and swallow their food and are called chewing insects.  All insects which chew the leaves of plants can be destroyed by putting poison on their food.  The common poisons used for this purpose are Paris green and London purple, which contain arsenic, and are used at the rate of one teaspoonful to a pail of water or one-fourth pound to a barrel of water.  This is sprinkled or sprayed on the leaves of the plants.  Another poison used is white hellebore.  This loses its poisoning