The reach for light is well marked wherever plants are crowded. About the edge of a forest, the trees branch on the outward side (Fig. 43). In the midst of the forest they shoot straight up. In the open field they branch on all sides and remain low. When two or three trees grow close together, they branch mostly in opposite directions These adaptations are equally marked in bushes and herbs.
Food.—The food requirements of plants are very different ; some grow best on poor soils, others on rich soils. In general, starving a plant causes it to flower and fruit more quickly but to produce a less abundant c crop. Over-feeding creates a tendency to produce stem and leaf at the expense of fruit.
It also greatly increases the tendency to produce monstrosities. Both these effects are especially produced by an over-supply of nitrogen. Abundance of water acts in much the same way as abundance of food. Over-supply of nitrogen may be corrected, to a certain extent, by the application of potassium, which tends to check the over production of vegetative parts and bring about the development of fruit. Some ex periments seem to indicate that phosphorus also directly favors fruit formation.
Lime is valuable not only as a food, but it helps to make other mineral food available. It hastens the decomposition of humus, sweetens sour soil and improves the texture of clay soils by its floccu lating action. It also acts as an antidote to the poisonous action of magnesium, when the latter is present in large quantities. Some plants are found only where lime abounds, while others cannot toler ate it except in small amounts.
If any nutritive substance in the soil be reduced to a minimum, the effect on the plant is much the same as if all the nutritive substances were like wise reduced ; this is known as the " law of the minimum." Consequently, the application of fer tilizer containing an element deficient in the soil, may give results out of all proportion to its cost.
It is possible in water cultures to determine very closely the effect of excluding various neces sary elements. For example, it is thus found that when iron is lacking, practically no chlorophyll is formed. The facts so gained have not as yet been applied to the soil to any great extent.
The root has a "selective action" in that it takes up from the soil certain elements, to the partial or total exclusion of others. Thus, from a
solution of sodium nitrate, it takes nitric acid, leaving the sodium. A cereal crop takes from the soil only one-fourth as much potash and only half as much nitrogen as root crops. This is one reason why a suitable rotation of crops is necessary to preserve the productiveness of the soil.
The physical condition of the soil is just as im portant as the chemical. It is almost useless to apply fertilizers to poorly tilled land. The food supply of the soil can be unlocked and made avail able to the plant only by judicious tillage.
Heal.—As previously stated, the most favorable temperature for the growth of plants is about 30° Centigrade (86° Fahr.). If the temperature rises much above this point, growth stops, and if the rise continues, death ensues. On the other hand, if the temperature is lowered, growth ceases before the freezing point is reached. Some plants may be frozen with impunity provided they are allowed to thaw out slowly. Others are invariably killed by freezing.
Too great cold and too great heat have much the same effect on the plant as lack of water. The former prevents absorption by the roots ; the latter causes water to evaporate from the leaves faster than it can be supplied. The habit of drop ping their leaves on the approach of cold weather, which deciduous trees have, is therefore compara ble to the action of desert plants in reducing their leaf surface.
In general, the plant that contains least water is most resistant to heat and cold. Dry seeds have been kept for a long time at the temperature of liquid hydrogen (— 238 ° C., or 396° F.) ; when thawed they grew normally. Bacteria are much more quickly killed by moist than by dry heat. Frost does not injure buds in winter when they are comparatively dry ; but in spring, when they are full of sap, it quickly kills them. The injurious action of frost is supposed to be largely due to the extraction of water from the cells by the forma tion of ice in the intercellular spaces. The air that normally occupies these spaces is thereby driven out, so that a frozen leaf, on thawing, resembles one in which the air of the intercellular spaces has been driven out by boiling. It is supposed that when the leaf is thawed slowly enough, the water is taken up again by the cells ; but when it is thawed quickly, the water escapes by evaporation before it can be reabsorbed.