The environment of the plant.
The needs of the plant are like those of the ani mal, namely, water, food, light, air and warmth. The plant resorts to endless contrivances to secure a sufficiency of these, as well as to protect itself against an excess of any of them. It constantly adjusts itself to external conditions in order to make the most of its circumstances. Were it not able to do so it would soon perish. We may briefly consider the chief factors of the environment.
Water.—Nothing affects the plant more than the water-supply. The effect of dry conditions is best seen on desert plants, which show the following modifications : (1) Reduced surface secured by partial or total suppression of leaves, as in cacti. The discarding of leaves in winter is an adapta tion to the dry conditions then obtaining. Even when there is water in the ground the roots can not absorb it, because of the low temperature ; (2) reduced evaporation secured by thicker epidermis, coverings of wax and of varnish ; (3) reduced evaporation secured by rolling the leaf, as in grasses, or placing it in a permanently vertical position, as in iris ; (4) storage of water in the thickened stem or leaf ; (5) reduction in the num ber of stomata and sinking them in depressions ; (6) hairy coverings of the leaves ; (7) increased woody fiber ; (8) smaller air-spaces ; (9) longer palisade cells of the leaf.
Aquatic plants show the opposite characteristics, having large surfaces, thin epidermis, no waxes, resins or hairs, very little woody fiber, very large air-spaces, and poorly developed palisade cells in the leaf. Stomata occur only on the surfaces exposed to air, but are there numerous.
The size of every part of the plant is increased by an abundance of water. The large-celled spring wood is an illustration of this. The small-celled fall wood is formed under much drier conditions.
Growing plants in a saturated atmosphere pro duce curious modifications ; a cactus may thus be made to produce leaf-like organs, and gorse pro duces leaves instead of thorns. On the other hand, a potato grown with a minimum amount of water, exposed to light, assumes a cactus-like habit, with no leaves and with very short internodes and thick ened stems.
The water-supply directly influences the produc tion of flowers and fruit. Aquatic plants cannot as a rule produce flowers under water. Land plants with abundant water-supply run to stem and leaf, and produce little fruit. Cutting off the water sup ply at the proper time greatly increases the pro duction of fruit, and also makes it sweeter and of higher flavor. By irrigating properly, we may con trol both the quantity and quality of the crop. An excess of water soon kills the plant by suffocating the roots.
Light.—The effect of light on the plant is very similar to that of dryness, and in the case of desert plants the strong light increases the effects due to lack of water. Plants that prefer the sun are known as sun-plants (grasses), while those that can grow only in shade are known as shade-plants (ferns). The latter have longer, thinner leaves, usually of paler color. A similar difference may often be observed between exposed and shaded leaves on the same individual plant. The exposed leaves have thicker epidermis, longer palisade cells, smaller air-spaces and fewer stomata.
Both leaves and branches arrange themselves with reference to the direction of the light, and the same is true to a large extent of flowers. This is well illustrated by plants that grow near houses so that they are shaded on one side. A further illustration is the different arrangement of leaves on upright and on horizontal branches of the same plant. Excessive light produces "sunscald " and other bad effects. Some leaves avoid this danger by assuming a vertical position. On the other hand, absence of light produces marked effects. Chief of these is the pale color (etiolation) which is so noticeable in celery that has been blanched by being covered from the light, or in potatoes that have sprouted in darkness. The stem is usually weak and spindling, while the leaves, in dicotyledons at least, remain small ; hairs and even prickles tend to disappear in darkness. With weak light the colors of flowers are much less brilliant, and the production of both flowers and fruit is seriously checked, even when there is sufficient food present for their formation.