If the tree be girdled the roots sooner or later suffer from lack of food supply from the leaves. Owing to this food stoppage the roots will cease to grow and will soon be unable to take in sufficient water, and then the leaves will begin to droop. This, however, may not happen until several months after the girdling. Sometimes a partly girdled branch grows much in thickness just above the girdle, as is shown in Fig. 29.
This extra growth seems to be due to a stoppage of the rich supply of food which was on its way to the roots through the bark. It could go no farther and was therefore used by the tree to make an unnatural growth at this point. You will now understand how and why trees die when they are girdled to clear new ground.
[Ill.u.s.tration: FIG. 29. A THICKENING ABOVE THE WIRE THAT CAUSED THE GIRDLING]
It is, then, the general law of sap-movement that the upward current from the roots pa.s.ses through the woody portion of the trunk, and that the current bearing the food made by the leaves pa.s.ses downward through the bark.
=EXERCISE=
Let the teacher see that these and all other experiments are performed by the pupils. Do not allow them to guess, but make them see.
Girdle valueless trees or saplings of several kinds, cutting the bark away in a complete circle around the tree. Do not cut into the wood. How long before the tree shows signs of injury? Girdle a single small limb on a tree. What happens? Explain.
SECTION XIV. THE FLOWER AND THE SEED
Some people think that the flowers by the wayside are for the purpose of beautifying the world and increasing man"s enjoyment. Do you think this is true? Undoubtedly a flower is beautiful, and to be beautiful is one of the uses of many flowers; but it is not the chief use of a flower.
You know that when peach or apple blossoms are nipped by the spring frost the fruit crop is in danger. The fruit of the plant bears the seed, and the flower produces the fruit. That is its chief duty.
[Ill.u.s.tration: FIG. 30. PARTS OF THE PISTIL]
Do you know any plant that produces seed without flowers? Some one answers, "The corn, the elm, and the maple all produce seed, but have no flower." No, that is not correct. If you look closely you will find in the spring very small flowers on the elm and on the maple, while the ear and the ta.s.sel are really the blossoms of the corn plant. Every plant that produces seed has flowers, although they may sometimes seem very curious flowers.
[Ill.u.s.tration: FIG. 31. A b.u.t.tERCUP]
Let us see what a flower really is. Take, for example, a b.u.t.tercup, cotton, tobacco, or plum blossom (see Figs. 31 and 32). You will find on the outside a row of green leaves inclosing the flower when it is still a bud. These leaves are the _sepals_. Next on the inside is a row of colored leaves, or _petals_. Arranged inside of the petals are some threadlike parts, each with a k.n.o.b on the end. These are the _stamens_.
Examine one stamen closely (Fig. 33). On the k.n.o.b at its tip you should find, if the flower is fully open, some fine grains, or powder. In the lily this powder is so abundant that in smelling the flower you often brush a quant.i.ty of it off on your nose. This substance is called _pollen_, and the k.n.o.b on the end of the stamen, on which the pollen is borne, is the _anther_.
[Ill.u.s.tration: FIG. 32. A PLUM BLOSSOM]
The pollen is of very great importance to the flower. Without it there could be no seeds. The stamens as pollen-bearers, then, are very important. But there is another part to each flower that is of equal value. This part you will find in the center of the flower, inside the circle of stamens. It is called the _pistil_ (Fig. 32). The swollen tip of the pistil is the _stigma_. The swollen base of the pistil forms the _ovary_. If you carefully cut open this ovary you will find in it very small immature seeds.
[Ill.u.s.tration: FIG. 33. STAMENS _a_, anther; _f_, filament]
Some plants bear all these parts in the same flower; that is, each blossom has stamens, pistil, petals, and sepals. The pear blossom and the tomato blossom represent such flowers. Other plants bear their stamens and pistils in separate blossoms. Stamens and pistils may even occur in separate plants, and some blossoms have no sepals or petals at all. Look at the corn plant. Here the ta.s.sel is a cl.u.s.ter of many flowers, each of which bears only stamens. The ear is likewise a cl.u.s.ter of many flowers, each of which bears only a pistil. The dust that you see falling from the ta.s.sel is the pollen, and the long silky threads of the ear are the stigmas.
[Ill.u.s.tration: FIG. 34. A TOMATO BLOSSOM]
Now no plant can bear seeds unless the pollen of the stamen falls on the stigma. Corn cannot therefore form seed unless the dust of the ta.s.sel falls upon the silk. Did you ever notice how poorly the cob is filled on a single cornstalk standing alone in a field? Do you see why? It is because when a plant stands alone the wind blows the pollen away from the ta.s.sel, and little or none is received on the stigmas below.
[Ill.u.s.tration: FIG. 35. CUc.u.mBER BLOSSOMS]
In the corn plant the stamens and pistils are separate; that is, they do not occur on the same flower, although they are on the same plant. This is also true of the cuc.u.mber (see Fig. 35). In many plants, however, such as the hemp, hop, sa.s.safras, willow, and others, the staminate parts are on one plant and the pistillate parts are on another. This is also true in several other cultivated plants. For example, in some strawberries the stamens are absent or useless; that is, they bear no good pollen. In such cases the grower must see to it that near by are strawberry plants that bear stamens, in order that those plants which do not bear pollen may become _pollinated_; that is, may have pollen carried to them. After the stigma has been supplied with pollen, a single pollen grain sends a threadlike sprout down through the stigma into the ovary. This process, if successfully completed, is called _fertilization_.
=EXERCISE=
Examine several flowers and identify the parts named in the last section. Try in the proper season to find the pollen on the maple, willow, alder, and pine, and on wheat, cotton, and the morning-glory.
How fast does the ovary of the apple blossom enlarge? Measure one and watch it closely from day to day. Can you find any plants that have their stamens and ovaries on separate individuals?
SECTION XV. POLLINATION
Nature has several interesting ways of bringing about pollination. In the corn, willow, and pine the pollen is picked up by the wind and carried away. Much of it is lost, but some reaches the stigmas, or receptive parts, of other corn, willow, or pine flowers. This is a very wasteful method, and all plants using it must provide much pollen.
Many plants employ a much better method. They have learned how to make insects bear their pollen. In plants of this type the parts of the blossom are so shaped and so placed as to deposit pollen from the stamen on the insect and to receive pollen from the insect on the stigmas.
When you see the clumsy b.u.mblebee clambering over and pushing his way into a clover blossom, you may be sure that he is getting well dusted with pollen and that the next blossom which he visits will secure a full share on its stigmas.
When flowers fit themselves to be pollinated by insects they can no longer use the wind and are helpless if insects do not visit them. They therefore cunningly plan two ways to invite the visits of insects.
First, they provide a sweet nectar as a repast for the insect visitor.
The nectar is a sugary solution found in the bottom of the flower and is used by the visitor as food or to make honey. Second, flowers advertise to let each insect know that they have something for it. The advertising is done either by showy colors or by perfume. Insects have wonderful powers of smell. When you see showy flowers or smell fragrant ones, you will know that such flowers are advertising the presence either of nectar or of pollen (to make beebread) and that such flowers depend on insects for pollination.
[Ill.u.s.tration: FIG. 36. BEES CARRYING POLLEN]
A season of heavy, cold rains during blossoming-time may often injure the fruit crop by preventing insects from carrying pollen from flower to flower. You now also understand why plants often fail to produce seeds indoors. Since they are shut in, they cannot receive proper insect visits. Plants such as tomatoes or other garden fruits dependent upon insect pollination must, if raised in the greenhouse where insects cannot visit them, be pollinated by hand.
=EXERCISE=
Exclude insect visitors from some flower or flower cl.u.s.ter, for example, clover, by covering with a paper bag, and see whether the flower can produce seeds that are capable of growing. Compare as to number and vitality the seeds of such a flower with those of an uncovered flower. Observe insects closely. Do you ever find pollen on them? What kinds of insects visit the clover? the cowpea? the sourwood? the flax? Is wheat pollinated by insects or by the wind or by some other means? Do bees fly in rainy weather? How will a long rainy season at blossoming-time affect the apple crop? Why?
Should bees be kept in an orchard? Why?
SECTION XVI. CROSSES, HYBRIDS, AND CROSS-POLLINATION
In our study of flowers and their pollination we have seen that the seed is usually the descendant of two parents, or at least of two organs--one the ovary, producing the seed; the other the pollen, which is necessary to fertilize the ovary.
It happens that sometimes the pollen of one blossom fertilizes the ovary of its own flower, but more often the pollen from one plant fertilizes the ovary of another plant. This latter method is called _cross-pollination_. As a rule cross-pollination makes seed that will produce a better plant than simple pollination would. Cross-pollination by hand is often used by plant-breeders when, for purposes of seed-selection, a specially strong plant is desired. The steps in hand pollination are as follows: (1) remove the anthers before they open, to prevent them from pollinating the stigma (the steps in this process are ill.u.s.trated in Figs. 37, 38-39); (2) cover the flower thus treated with a paper bag to prevent stray pollen from getting on it (see Fig. 40); (3) when the ovary is sufficiently developed, carry pollen to the stigma by hand from the anthers of another plant which you have selected to furnish it, and rebag to keep out any stray pollen which might accidentally get in; (4) collect the seeds when they are mature and label them properly.
Hand pollination has this advantage--you know both parents of your seed.
If pollination occur naturally you know the maternal but have no means of judging the paternal parent. You can readily see, therefore, how hand pollination enables you to secure seed derived from two well-behaved parents.
Sometimes we can breed one kind of plant on another. The result of such cross-breeding is known as a _hybrid_. In the animal kingdom the mule is a common example of this cross-breeding. Plant hybrids were formerly called mules also, but this suggestive term is almost out of use.
[Ill.u.s.tration: FIG. 37 The bud on right at top is in proper condition for removal of anthers; the anthers have been removed from the buds below]
It is only when plants of two distinct kinds are crossed that the result is called a hybrid; for example, a blackjack oak on a white oak, an apple on a pear. If the parent plants are closely related, for example, two kinds of apples, the resulting plant is known simply as a _cross_.
Hybrids and crosses are valuable in that they usually differ from both parents and yet combine some qualities of each.
[Ill.u.s.tration: FIG. 38. ORANGE BLOSSOM PREPARED FOR CROSSING First, bud; second, anthers unremoved; third, anthers removed]
[Ill.u.s.tration: FIG. 39. TOMATO BLOSSOM READY TO CROSS First, bud; second, anthers unremoved; third, anthers removed]
[Ill.u.s.tration: FIG. 40.
First, blossom bagged to keep out stray pollen; second, fruit bagged for protection]