"How often shall I water my plants?" asks the purchaser of a small bill at the nursery. In window gardening the water question is also one of the anxious ones - and even in the regular operations of gardening, under the treatment of quite practiced hands, the relations of water to plant life is not as clear as it might be.

We shall understand better how to water if we correct first some impressions derived from old works on physiology. It is said that plants want water. This is not strictly true. Water is found in plants, but it enters rather in the shape of vapor.

A soil that is wet will grow only water plants; and it is a remarkable fact that these water plants seem to have very little water in them. A reed or bulrush grown in water has far less water in its structure than a nearly allied species grown on the dry land. The plants which have most fluid matter in them are those grown in the dryest places. The deserts of Africa abound in Euphorbias; while on the plains of Mexico the only moisture wild cattle can often get is from the large spiny Globe Cactuses, which they manage to cleave open with their hoofs.

A wet soil is totally unfit for plant growing. A plant standing twenty-four hours in water is often irreparably injured. A Hyacinth, to be sure, will live one season in water; but all the matter which goes to make up the flower is prepared the year before, and after flowering the bulb is exhausted and almost worthless.

A good soil for plant growing, therefore, is not one which will hold water; but one in which water will rapidly pass away.

The soil itself is composed of minute particles, through which air spaces abound. The water must be just enough to keep these particles moist, and the air in the spaces is thus kept in the condition of moist air. The roots traverse these air spaces, and it is therefore moist air which roots want, and not water.

If it were water simply which plants wanted, we should cork up the bottom of the hole in the flower pot, and prevent the water getting away. Instead of this we try to hasten the passing of the water through as much as possible; by not only keeping the hole as clear as possible, but often by putting pieces of broken material over the hole.

A plant will generally be the healthiest, therefore, which wants water the oftenest. This will show that there are plenty of air spaces, and that the roots are making good use of them. If it does not often want water it is in a bad way, and more water will make it worse.

How often to water them will be according to how easy the water passes away. If when you pour water on earth it disappears almost instantaneously, it would be safe to water such plants every day.

The greenhouse will now begin to look more natural, after having had the stock housed last month. With many plants having probably been taken up out of the open ground, dead leaves will daily appear, requiring frequent removal. Neatness is one of the chief beauties of a greenhouse. Acacias, and Australian plants generally, with hard wood and delicate roots, should be placed at the coolest end of the house, where little water will be required. These plants should not be watered often; but when they are, it should be thorough. Frequent waterings soon render the roots of these plants unhealthy, when it is very difficult to restore them to vigor. Whenever the foliage becomes of sickly yellow hue, the best plan is to plunge the plant in a larger pot, filling the space with moss - and when the plant requires water, give it only through the moss, unless the plant seems to become so dry as to suffer, when it should receive one thorough watering. Very little fire should be applied to a greenhouse - just sufficient to keep it at about 450. Unless very far north, but little fire-heat will be required this month.

Window plants should not be kept very warm at this season. They should have all the sun and air, and as little of the artificial heat of the room as possible. These remarks apply especially to Mignonette, which is very impatient of in-door confinement. Succulents, such as Cacti, are excellent window plants in this respect, as the dry air does not affect them. To keep the air about the plants moist, is one of the secrets of window culture. Some who have very fine windows well stocked with fine plants, make glazed cases with folding doors of them, by which, when the room is highly heated and very dry, they can be enclosed in an atmosphere of their own. In such cases, Ferns and Mosses can be grown to perfection, and pendant plants in hanging vases give a Brazilian forest appearance to our happy Christmas homes.

Hanging baskets, on the other hand, are generally too dry. Besides the daily waterings, about once a week they should be immersed in a bucket of water.

Plants stored away for the winter in cold pits, require more care for the first month or so than at any other time through the winter season. Many of them have unripened shoots, or shed many of their leaves, and unless they be cut off and removed, gangrene and decay commit distressing havoc. Air should be given at every opportunity, and nothing omitted that will, in any way, tend to harden the plants, and send vegetation to rest. No more water should be given than just sufficient to prevent withering, and the temperature should be kept as near 400 as possible, and every chance taken to render the air about the plants dry. When frost actually does come, no further care than protection from its embraces will then be required. Plants so hardened may stay covered up for weeks, without any light or air, and secure from the slightest injury. Mice constitute the most troublesome enemy in a pit closed for any length of time; but we have, as yet, found nothing better than the recommendation given in back volumes, namely, to take Peas and soak them twenty-four hours in water, then roll in arsenic and sow in a pot, as if in the regular way of seed-sowing. A few pots so prepared should be placed in the pit before permanently closing up.

The mice usually make for these pots at their first entrance to the pits. If placed on the soil, they seem to guess your secret, and will not "bite".

Plants in cellars need much the same care as those in pits. Avoid heat and dampness; frequently however, plants suffer in cellars through getting too dry. They should be looked over, at any rate, once a month, and a little water given, if likely to become entirely dry.