200 Pounds of water

have passed through

the plant. Measure-

ments of transpiration

show that about 300

pounds of water are A^

required for one pound ^9^

of growth in corn and Vat

about 500 pounds for F'"- H— Comparison of water used with

, , ,, . wheat produced. (After Widtsoe.)

one pound of growth m

wheat (Fig. 14). A pint of wheat weighs a pound, but 60

gallons of water are necessary to produce it. If the straw

weighs as much as the grain, three 40-gallon barrels full of

water are transpired in growing the pint of wheat. This

quantity of water used in growing a pound of dry substance

is called the water-cost of dry matter for the particular plant.

48 The Principles of Agronomy

Crops growing in hot, sunshiny regions transpire more

water than in humid regions. Dry air, winds, poor soil,

weak plants, and an abundance of water in the soil cause

more water to be used for dry matter produced. Desert

plants and drouth-resistant crops have the power to hold

so much water in their tissues against the forces of tran-

spiration that they do not die from wilting. Some plants

also have the power of developing few or many stomata

according to whether they have small or large quantities

of water at their disposal. Some plants transpire much

more water than others ; most plants seem to be wasteful

during the period of bloom ; and quick-growing crops use

more water than steady-growing ones. Darkness also

diminishes transpiration considerably.

35. Translocation. — Since all the starch and other

plant-food is elaborated in the leaves, this must be moved

or the leaves would be the largest part of the plant. En-

zymes change sugar into starch for storage, and then to

sugar again when moving is begim. The solubility of

sugar allows the sap stream to carrj^ it to the fruit, stem,

or root for use or for storage. For example, great quan-

tities of sugar or starch are stored in roots of carrots to

be moved to the flowers and seed when the plant matures

and* seed is set. Fruit trees move food from wood to the

fruit. Seed and fruit often grow so rapidly that storage

in early summer is necessary. This movement of elabo-

rated food from one part of the plant to another is called

translocation. Most crop plants become more or less

porous in stem or roots, or in both, during the seed-setting

period owing to the transfer of food material.

36. Transportation. — Water is transported upward

through the tracheal tubes and sap, downward through

the sieve tubes, or radially along medullary rays. Sap

can flow down largely by gravity and radially by capil-

Plant Functions 49

larity, or wick action. Both these forces aid osmotic

pressure in forcing the water upward. Whatever factors

are at work, the water seems to have little more trouble

in reaching leaves on a tree-top 200 feet above ground

than those on a strawberry plant.

A geranium-cutting with the cut end immersed in

red ink will soon show red stains moving upward. They

will finally extend along the leaf-veins causing red blotches

in the leaves where the liquid is released into the spaces

between the cells. A plant stem several inches in length

will be traversed in a few minutes. The rate of movement

varies from a few inches to several feet an hour.

37. Response. — Nearly all plants tend to grow verti-

cally, even on a steep hillside where it would seem that

growth at right angles to the slope would afford the

firmest root attachment. Most plants in windows lean

toward the light, and must be turned every few days

to prevent their becoming one-sided. Roots, in wet soils,

nearly always grow in the surface layer, while on dry,

well-drained soils, they penetrate deeply. It is coimted

a good practice to withhold irrigation water as long as

possible in order to promote deep rooting. Oxygen as well

as water limits the growth in swamped soils. Many

plants do not thrive save on soils rich in lime. Alfalfa

is a notable example.

SUPPLEMENTARY READING

Any textbook of botany.

Plant Physiology, B. M. Duggar.

Plant Physiology, L. Jost.

Plant Physiology, G. J. Peirce.

Cyclopedia of American Agriculture, Vol. II, pp. 11-22.

B

CHAPTER V

THE PLANT AS A FACTORY

Notwithstanding their complexity, plants are simple

in their purpose if they can be said to have such. All their

energies are bent toward seed production or toward some

other means of continuing the species, that is, of transmit-

ting life and characteristics to another generation. Sin-

gleness of aim seems to show the organization of the plant

and the variety of ways in which it attempts to preserve

itself in the struggle for existence. Power to gather

raw foods and elaborate them into tissue-building com-

pounds, storage of these products in some part of the

plant against the time of greater needs, and adjustment

to surroundings are nothing more than manifestations

of the struggle to perpetuate the species.

In the products of the plant, man is vitally interested.

Sometimes it is the roots, sometimes the stem, the seed,

or the fruit containing stored starch, sugar, oil, or pro-

tein that draws his energy in producing and harvesting.

Often it is just the dead cell-walls, such as wood, cork, or

straw ; but other times he takes the plant in the midst

of life to get immature steins for forage, sap for rubber or

turpentine, or cell-contents for sugar. Every part of the

plant has been put to use ; roots, stems, leaves, flowers,

seeds, and sap, all furnish useful products. For example,

beets and carrots are roots, hay is both stem and leaf,

grain and beans are seeds, some perfumery is made of

blossoms, and cane-sugar is a sap product. Drugs and

50

The Plant as a Factory 51

stimulants, such as opium, tobacco, and quinine, come

from substances known as alkaloids that may be found in

any part.

38. Interdependence of plants and animals. — If only

plants were upon the earth, then the provision of nature

for plants to give up oxygen and use carbon dioxide and

for animals to reverse these processes, would be useless.

Animals feed upon plants, directly and indirectly ; directly

when they are plant-eaters, and indirectly when tliey are

flesh-eaters; for the prey of carnivorous animals either

ate plants, or animals that ate plants. On the other

hand, decayed bones, flesh, and manure restore to the

sofl and air substances upon which the plants feed either

directly or indirectly ; directly when plant-food is at once

taken from the broken-down tissues, and indirectly when

these decaying substances promote the growth of soil bac-

teria which take atmospheric nitrogen and make compounds

that the plant absorbs. Soil devoid of organic matter —

decaying plant and animal substance — is almost useless

on account of its being compact. It can hold water

for only a short time ; air and heat cannot pass through

it readily.

Many plants require limestone soils for development.

Part of the limestone ledges supplying lime is composed

largely of shells of small animals that extracted lime from

the water in which they lived. These animals probably

fed on water plants, and breathed oxygen released as

by plant processes. In the economy of nature, plants

and animals need each other.

39. Dependence of man on plants. — Since animals

depend on plants for their food, man, who in turn depends

on plants and animals, may be regarded as being ulti-

mately dependent on plants. It is not difficult to see

that almost all hiunan food, save only a few minerals

52 The Principles of Agronomy

such as salt, comes entirely as a result of life processes.

Milk, cheese, butter, flesh, and eggs are body products

of animals ; bread, fruit, vegetables, and " greens "

are plant contributions. Furthermore, they are produced

almost entirely by domesticated plants and animals.

Clothing, likewise, comes largely from the same sources.

Cloth made from wool, hair, cotton, or flax fiber is just

as truly the product of animals and plants as are the skin

garments of the Eskimo or the leaf and bark raiment of

the tropical savage; furs, gloves, shoes, and straw hats

are made directly from products of the life processes.

Formerly, most dwelling places were built of wood and

leaves or of skins. Modern buildings consist largely of

brick, stone, concrete, and metal, but wood is used in

lathing, for floors, door and window frames, for roofing, and

for walls in many cases. Furniture and useful tools will

for years to come, if not always, be composed largely of

wood. Attention to forestry indicates that man realizes

this and is making an effort to preserve his timber re-

sources. Moreover, far back in the history of cement,

brick-clay, some rock, and some metals, life has played

a part. Especially is this true wherever carbonate com-

pounds exist, since others as well as limestone have re-

sulted from an organic process somewhere in the chain of