CHAPTER IX. FRUITS: HOW SEEDS ARE SCATTERED.

35. PLANTS WHICH SOW THEIR OWN SEEDS.

1. =The fruit of the wallflower.=—Examine wallflower fruits and make out that each consists of the ripened pistil. Does the fruit open of itself? How many chambers does it consist of? Where are the seeds attached? Are they blown off at last by the wind? Draw the fruit. It is called a _siliqua_.

2. Compare with this the fruit of _shepherd’s purse_ (Fig. 62), and _penny cress_ (Fig. 132), and notice that they are of the type of the wallflower fruit, but are much broader in proportion to the length. Such a fruit is called a _silicula_. Draw.

3. =The fruit of the pea.=—Examine a ripe pea-pod and compare it with (_a_) the pistil of an unfertilised flower, (_b_) a half-ripe pod. How many carpels have taken part in forming the pod? How many seeds (peas) does the pod contain? Leave a pod on the plant until the shell becomes dry, to find out how the fruit opens. Does it open along one edge only, or along both? How are the seeds attached? Such a pod is called a _legume_. Draw it.

Compare and draw the legumes of the _broad bean_, _French bean_, _scarlet runner_, _laburnum_ (remember its seeds are poisonous), and _bird’s foot trefoil_. The legume of the bird’s foot trefoil bursts open suddenly and scatters the seeds in the air. Is the scattering of the seeds any advantage? Why?

4. =The fruit of the field geranium.=—Make out that five carpels are grouped around a central rod. Examine fruits which have opened. About noon on a bright, sunny day gently touch a ripe fruit with a small brush, and watch the carpels spring back from the rod and jerk the seeds into the air. Compare and contrast this fruit with a siliqua, silicula, and legume respectively.

5. =The fruit of the poppy.=—Examine a poppy head. The top of the fruit is the stigma. Observe below this a line of small holes running round the fruit. Draw. Shake the fruit, and notice that seeds fall out through the holes. Cut the fruit across to see the large number of small seeds inside. How does the fruit hang on the growing plant? Does the wind shake it and liberate the seeds?

6. =The fruit of the pansy and violet.=—Watch the ripening of the fruits on the plants. Observe that the ovary swells up into an egg-shaped body which afterwards splits into three boat-shaped valves containing seeds. Try to make out why the seeds are one by one shot out as the sides of the valves dry. Put a ripe fruit before the fire and watch the process. Imitate it by placing a pea between two flat rulers and pressing the rulers together.

=The origin of a fruit.=—When the ovules of a flower have been fertilised (p. 92) by the pollen tubes they change into seeds which have the remarkable power of growing up—in favourable circumstances—into plants resembling that which produced the seeds. This is not, however, the only result of fertilisation. Whilst the ovules are changing into ripe seeds, those parts of the flower—the stamens, corolla, and calyx—which have finished their work wither and fall off, though the calyx sometimes remains. Other parts—the pistil and sometimes the receptacle (p. 90)—take on new duties, and become gradually modified in order to protect or scatter the seeds.

Thus, the tender wall of the pistil often becomes a woody, leathery, or juicy seed-case; while the receptacle, or top of the flower stalk, may become fleshy and swollen with sugary pulp, as a bait for birds and other animals. In any case we give the name of fruit to all such altered and persistent parts together with the seeds which accompany them. A pea pod, for example, is as truly a fruit as a plum, and a poppy-head as a strawberry.

What a fruit is like depends to a great extent upon the characters of the pistil which gave rise to it. If, for example, the pistil consists of several separate carpels, the ripened carpels or fruits will also be separate. When, on the other hand, the pistil is composed of united carpels, these will remain united, at least until the fruit is ripe. Then in some cases they come apart.

The part of a fruit which is derived from the walls of the pistil is called the =pericarp=.

=The fruit of the wallflower.=—After fertilisation the stamens, petals, and sepals of the wallflower drop off, leaving the pistil alone on the top of the flower stalk. The pistil increases greatly in size (Fig. 131, _B_) during the ripening of the seeds. At last its wall (the pericarp) splits into two flaps. These become free at the bottom, exposing a central plate which bears the rows of seeds. When the seeds are quite ripe, a slight breeze is sufficient to shake them off, and they fall to the ground to take their chance of finding a place favourable for germination.

A fruit with a dry pericarp, which opens of itself when the seeds are ripe, is called a =capsule=. This particular kind of capsule—consisting of two carpels which come apart at maturity, leaving a central partition bearing seeds—is known as a =siliqua=. When it is short in proportion to its length, as in the shepherd’s purse (Fig. 62) and penny cress (Fig. 132), it is distinguished as a =silicula=.

=The fruits of the pea tribe.=—The pod of the pea (Fig. 3) and its relatives is a capsule of another kind. It consists of one carpel only, and opens, when ripe, along both back and front margins to liberate the seeds. Such a fruit is called a =legume=. In the young fruit the pericarp is somewhat fleshy and succulent, but it becomes dry during ripening. The legume of the bird’s foot trefoil bursts open suddenly, and throws the seeds to a considerable distance.

=The fruit of the field geranium= (Fig. 133) is a long capsule composed of five carpels arranged round a central column (_A_). When the seeds are ripe, the carpels suddenly spring from the rod, remaining attached only at their upper ends (_B_), and fling the seeds into the air. The method may be watched by stroking the fruits very gently with a small brush, when they open in the manner described. The experiment is most likely to succeed in dry, sunny weather, about the middle of the day.

=The fruit of the poppy.=—The pistil of the poppy swells during ripening into a large, globular capsule known as the poppy head. The top of the fruit (Fig. 134) is the persistent stigma. Just below this, a line of small holes like windows runs round the head. As the fruit hangs inverted on the top of the flower stalk it is shaken about by the wind, and the tiny seeds fall out through the windows.

=The fruit of the violet or pansy.=—The arrangement by which the violet (Fig. 134) or pansy sows its seeds is most interesting. After fertilisation the ovary swells into a great egg-shaped capsule, in the inside of which the seeds are arranged in three rows. When the seeds have become ripe and hard, the capsule splits down the side along three lines, and is thus divided into three parts. These open outwards, and bend back as shown in Fig. 135. Each is a boat-shaped valve. The seeds inside are thus exposed to the air, and they soon dry and become ready for scattering.

Then the curved sides of each valve begin to straighten and come together, and naturally allow less and less room for the hard, smooth seeds inside. The pressure of the sides of each valve on the seeds inside it becomes greater and greater, until one by one they are shot out to a considerable distance. If a ripe violet ovary be warmed before the fire the whole operation may easily be watched. The process may be imitated by putting a pea between two flat rulers and pressing the rulers together, when the pea may be shot to a distance of several yards.

=Why fruits are scattered.=—A flowering plant is practically confined for life to the place where it first sprang up, so that it is unable to go about and select favourable situations for its offspring. On the other hand, if the seeds simply fell to the ground beneath the parent plant, the seedlings would generally be so crowded together that they would interfere very much with each other’s growth. In addition, they would often be under a great disadvantage, because the parent plant would keep so much light from them. Hence, very many plants have some special arrangement for scattering their seeds, so that some at least of the seeds will have a chance of falling in a place where they will obtain plenty of light, air, and good soil.

In this section we have studied examples of devices by which plants sow their own seeds. We shall see next that other plants call in the help of the wind and of animals.

36. SEEDS SOWN BY THE WIND.

1. =The fruit of the dandelion.=—Examine the manner in which the tiny flowers of the dandelion are grouped together to form the head (p. 113); and make out the various parts of the flower, especially the _ovary_—a little white knob at the bottom of the flower, and the _calyx-tube_—forming a tuft of fine hairs above the ovary. Trace the development of the fruits: the withering of the corollas and stamens, the elongation of the calyx-tube, and the expansion of the tuft of hairs to form a parachute. Blow a dandelion “clock” (the head of fruits), and notice how the parachutes float the fruits in the air.

Examine “thistle down” and contrast it with the dandelion fruit.

2. =The fruit of the willow.=—Examine the ripened catkins of a female willow in June, and notice how each fruit has split into halves, which come apart and expose the silky seeds inside. Pull a tuft of seeds out and dry them in the sun. Notice that they wriggle and writhe about and gradually become entangled together into a woolly mass, which is easily blown away.

3. =The elm fruit.=—About the end of April look for elm fruits. Notice the flat green plate (wing) with the rounded swelling near the middle (Fig. 124, 5). Cut open the fruit and see that the swelling is caused by a single seed. How does the flat wing aid in the distribution of the seed?

Compare the winged fruits of the _ash_ and _sycamore_ (Fig. 137).

Do these winged fruits drop from the trees easily, or are they torn off by gales?

Take a pair of sycamore fruits. Cut off the wing from one and let it and an uninjured fruit fall at the same instant from a height. Which reaches the ground first? Why?

4. =Pine and fir cones.=—Examine and compare pine and fir cones of various ages. Break open a ripe cone and see the scales with the pairs of naked seeds, each seed bearing a thin, papery wing which has split off from the upper surface of the scale.

=Wind-sown seeds.=—Many plants depend on the wind for the dispersal of their seeds, and consequently the seeds are provided with outgrowths of various kinds, which increase the surface greatly without adding much to the weight, and, acting like parachutes, offer increased resistance to the air and thus prevent the seeds from falling quickly to the ground. In some cases the outgrowth is part of the pericarp, in others it is an appendix carried by the seed itself, while in the lime it is the bract upon which the flowers were formerly borne.

=The dandelion fruit.=—The fruit of the dandelion (Fig. 136) affords one of the best possible examples of wind-dispersal. It will be remembered (p. 113) that what is commonly called the flower of the dandelion is really a head of perhaps 300 complete flowers: each with a hairy calyx-tube, a yellow, strap-shaped corolla, five stamens, and a pistil. When the flowers have been fertilised, the yellow corollas and the stamens wither, the ovary increases in size with the ripening of the single nutlet in its interior, and each calyx-tube elongates until it is about an inch in length, the tuft of fine hairs being still at its upper end. The attachment of the fruit (Fig. 86, 4) to the disc (receptacle) is so slight when the seed is ripe that a very gentle puff of air is sufficient to overcome it. The tuft of hairs at the upper end has by this time expanded until it acts like a parachute, which supports the tiny fruit for a long time in the air.

The common =thistle=—a relative of the dandelion—also distributes its fruit by means of a tuft of fine hairs derived from the calyx. In this case the hairs radiate from the seed. Such “thistle down” is commonly found floating through the air in summer.

=The willow fruit.=—The catkins of the female willow are ripe in June. Each catkin consists of a large number of tiny pods derived from the ovaries of the flowers (p. 151). Each fruit splits into halves, which bend back from each other (Fig. 121, _F_), exposing the silky seeds within to the warmth of the sun. The seeds (_H_) turn and twist about as they dry, and gradually entangle themselves together into a light, woolly mass, which is easily blown to great distances by the wind. The willow and the dandelion, therefore, use very similar devices to ensure the dispersal of their seeds, although these plants are not at all nearly related.

=The fruits of the elm, sycamore, and ash.=—These common forest trees bear fruits with the seed attached to a flat plate, which is an outgrowth of the pericarp (p. 167). In the elm fruit (Fig. 124, 5) the plate is green and oval, and the seed forms a rounded swelling at, or near, its middle. The fruits of the =sycamore= generally grow in pairs (Figs. 33 and 137). Each half consists of a single seed with an attached membranous plate, and the two seed-boxes of each pair are in contact. The fruits of the =ash= hang from the twigs in bunches called “keys,” each fruit on a separate little stalk. The plates which bear the seed are long, narrow, and oval in shape.

It is plain that such plates, or wings, expose a relatively large surface to the air and prevent the fruit from falling to the ground as quickly as it otherwise would. The fruit is thus often blown to a great distance before it finally settles and the seed germinates. The action of the wing may be well shown by cutting off the plate from a sycamore fruit and letting it and one with an attached wing fall at the same instant from a height. The seed without a wing comes straight down as a pea would; but the winged seed spins in the air and settles more slowly.

In the case of the round downy fruits of the =lime=, a similar service is performed by the bract (Fig. 126), upon which the flowers were carried.

=Pine and fir cones.=—If a ripe pine, or fir cone is broken open, it will be seen that each seed is attached to a thin, papery wing (Fig. 130, 4), which has split off the upper surface of the scale bearing the seeds. The winged seeds are shaken out of the cone by the wind, and blown away.

=Trees alone bear winged seeds.=—Winged seeds would be useless to any but fairly high trees, because if they were formed on the low plant they would fall to the ground long before the wind could catch them properly. It is also interesting to find that such seeds are generally attached so firmly that they are only broken off by gales strong enough to carry them a considerable distance.

On the other hand, the tiny plumed fruit of the dandelion or thistle is so very light in comparison with the surface exposed to the air that it takes quite a long time to fall even a few inches.

37. SEEDS WHICH ARE SCATTERED BY ANIMALS.

1. =Hooked fruits.=—Examine plants of _herb bennet_ (wood avens) in summer and autumn, and find the fruits. Brush your sleeve against the fruits, and notice how they cling to the cloth. Examine them with a lens and observe the hooks at the ends of the styles.

Compare _goosegrass_ (cleavers) and find the hooks on the fruits.

2. =Nuts.=—Examine a _hazel_ nut. Notice the sheathing bracts at the base of the fruit. Crack the nut and examine the broken edge of the shell (pericarp) with a lens. Make out the three layers which compose it. How many seeds are present? Cut the seed (kernel) across and see that the bulk of it consists of two cotyledons (Chap. I.). Does the fruit open of itself, if undisturbed?

Compare the acorn of the _oak_. Trace the development of the acorn from the female flower, noticing that the cup is developed from a wrinkled disc surrounding the lower part of the flower. Cut across the ovary in June and notice that there are six ovules in it. In the ripe fruit observe that the cup separates easily from the nut. Remove the shell (pericarp) of the nut. How many seeds does it contain? What do you think has become of the other five ovules? Cut through the seed and observe the two cotyledons.

Compare the fruits of the _beech_. Notice that they are three-sided nuts (each being a seed enclosed in a woody pericarp); and that two nuts occur together, surrounded by a bristly woody cup which splits, when the nuts are ripe, into four valves.

3. =Stone fruits.=—Examine a ripe _plum_. Cut it open and crack the stone to see the single seed. Notice that the pericarp consists of three layers as in the hazel nut, but that here the middle layer is soft and fleshy, and the outer layer is the “skin.” The stone is the inner layer of the pericarp. Is there any special means of liberating the seed? Compare the _cherry_, and examine the seed and the three layers of the pericarp.

Examine the fruits of the _blackberry_ and _raspberry_, and observe that each consists of several small stone-fruits arranged on the receptacle.

4. =Berries.=—_The gooseberry._—Notice the stalk at the bottom of the fruit, and, at the top, the withered remains of the calyx. Cut across a half-ripe gooseberry and observe the thick, fleshy pericarp enclosing the seeds. Treat a ripe gooseberry in the same way, and observe that the pericarp has now for the most part become a soft pulp, in which the seeds are embedded. The rest of the pericarp is a membranous skin. Is there any special means of liberating the seeds?

Compare _grapes_, _currants_, _oranges_, and _vegetable marrow_ fruits, and notice that the structure of these resembles that of the gooseberry.

5. =The apple.=—Cut across the receptacle of the flower (apple blossom) and notice how the five carpels are buried in it (p. 106). Trace the formation of the fruit, and see how the _receptacle_ becomes larger and larger during ripening. At the top of the ripe fruit observe the withered remains of the calyx. Cut the apple through across the middle to see the _core_. This consists of the horny walls of the five carpels and the contained seeds (pips). From what part of the flower is the fleshy, eatable part of the apple derived? Compare the _pear_.

6. =The rose hip.=—Examine fruits of the wild rose. Notice that they are urn-shaped. On the flat rim of the urn observe the five scars left by the sepals (or, in some cases, the sepals themselves). In the opening of the urn see a tuft of greyish hairs. Cut the fruit down from top to bottom through the middle to see the thick, fleshy wall of the urn and the contained _nutlets_. From what part of the flower is the fleshy wall of the hip derived? Examine a nutlet. What does the tuft of hairs seen in the mouth of the urn consist of? Open a nutlet with a needle and pick out the seed.

7. =The strawberry.=—Cut a strawberry down the middle and notice at the base the persistent calyx, in the inside the fleshy receptacle, and on the outside the yellowish nutlets. Open a nutlet with a needle and pick out the seed.

=The help of animals.=—It has been seen in Chapter VI. that insects play a very important part in the fertilisation of many flowers. Very many plants also call in the aid of animals at a later stage for the dispersal of the seeds, and the devices by which this aid is obtained are often very ingenious.

=Hooked fruits.=—Sometimes after a country walk the reader has probably found small fruits and seeds sticking to his clothes. They have become attached to the cloth by means of small hooks which they carry. The fruit of the =herb bennet= (wood avens) is a good example of this device. When the stigma of the fruit breaks away, a little hook is left at the top of the style (Fig. 138). Goosegrass or cleavers—a common hedgerow plant—and many others have also hooked fruits. Sheep or cattle, grazing near such plants are very likely to brush against them and carry off the fruits and seeds in their hair. They may not again reach the ground until they have been carried far from the place where they grew.

=The position of hooked fruits.=—It is evident that these little hooks would be quite useless if the fruits grew out of the reach of animals. Such hooked fruits are never found, for instance, on high trees.

=Nuts.=—A fruit which has a dry, woody pericarp and does not open of itself is called a =nut=. The fruits of the hazel, oak, and beech are good examples. The shell (pericarp) of the =hazel= nut (Fig. 139) is composed of three layers, and encloses a single seed, or kernel. The nut of the =oak= (Fig. 113) is called an acorn. Its lower part lies in a cup developed from a wrinkled disc by which the lower part of the female flower was surrounded. When the fruit is ripe the nut easily separates from the cup. The acorn contains one seed only, which consists largely of two swollen cotyledons. If the ovary of the female flower of the oak is cut across in June six ovules are found in it. As in the case of the hazel only one ovule is allowed to reach maturity; the rest are sacrificed in order that the remaining one may be more perfectly developed. In the =beech=, two three-sided nuts occur together, surrounded by a bristly, woody cup, which splits into four valves when the nuts are ripe.

=The dispersal of nuts.=—=Squirrels= and other nut-eating animals are instrumental in the dispersal of the seeds in a somewhat indirect manner. They have a habit of storing up nuts and seeds in holes; but in their active life these animals often forget where their larder is. The seeds, thus left to themselves, sprout and grow into trees. The unripe nuts of the beech are protected from the attacks of squirrels by the hard bristles on the outer husk.

=Stone fruits.=—A ripe =plum= (Fig. 140) or =cherry= (Fig. 77), like a nut, consists of a seed and a pericarp; but here the pericarp is specially modified to tempt animals and at the same time to protect the seed from them. The inner pericarp-layer is a hard, woody shell called the stone; the middle part swells up to form a juicy, sweet mass; while the outer layer constitutes the “skin,” and is often beautifully coloured.

=Blackberries= (Fig. 141) and =raspberries= consist of several small fruits of the plum type, which are arranged round an axis derived from the receptacle of the flower.

=Berries.=—The =gooseberry= (Fig. 142) is a type of this class of fruit. It contains several seeds. The pericarp is thick and fleshy before the fruit is ripe, but during ripening the greater part of it becomes a soft, sweet pulp in which the seeds are embedded. The rest of the pericarp is a membranous skin. =Grapes=, =currants=, =oranges=, and =vegetable marrow= fruits are also berries. The vegetable marrow has obviously a general resemblance to a half-ripe gooseberry but is on a much larger scale.

=The apple.=—In the apple (Fig. 143) we have an example of a fruit in the formation of which the receptacle has taken a large share. Even in the flower, the five carpels of the pistil are buried in the receptacle, and as the seeds (pips) ripen, the =receptacle= swells until it composes the greater part of the fruit and at last becomes sweet and fleshy. The carpels with the contained seeds constitute the =core=. The withered sepals are still to be seen at the top of the fruit. The structure of the =pear= is quite similar to that of the apple.

=The fruit of the wild rose.=—The fruits of the wild rose are called =hips=. They are urn-shaped, and on the flat rim of the urn five scars show the former position of the sepals.

In some cases the sepals themselves have remained. The narrower mouth of the urn is filled by a tuft of greyish hairs which, when the fruit is cut open, are seen to be the styles of the carpels. The carpels are hairy, and stand on the bottom and sides of the urn-cavity. Each carpel contains a seed. Comparison with the flower shows that the red, fleshy urn is the developed =receptacle=.

=The strawberry.=—In the strawberry (Fig. 144) the eatable part of the fruit is again the swollen and juicy receptacle. In this case it has grown up on the _inside_ of the carpels—the little, yellow nutlets (carpels) lying on the surface. Each carpel contains a seed.

=Why some fruits are sweet.=—The delicious flavours of sweet fruits have been developed as baits for the allurement of such animals as are likely to scatter the seeds to the best advantage. Consider such fruits as cherries, currants, and rose-hips. Birds—such as thrushes—find them very nice to eat, and are ready to carry them away to consume at their leisure. The birds eat the sweet, fleshy part, but in the case of a stone-fruit they drop the stone and leave the seed to germinate. A seed which is in danger of being swallowed is either hairy like the rose-carpels (and therefore rejected because it irritates the mouth unpleasantly), or it is enclosed in a hard or horny case, upon which the animal’s digestive juices are unable to act. When the seed is dropped it is no worse for its experience, and with good luck grows up into a plant.

=Eatable fruits generally conspicuous.=—In order that birds and other animals may easily find the luscious fruits which they may eat as a reward for scattering the seeds, such fruits are almost always displayed very conspicuously, and are brilliantly coloured. Oranges, plums, red currants, apples, etc., illustrate this fact well. =Red= is perhaps the commonest colour of eatable fruits, because it contrasts so strongly with the green colour of the foliage (Fig. 145).

EXERCISES UPON CHAPTER IX.

1. Name and define the different kinds of self-opening fruits. From what common plants could you collect examples of these during an Autumn walk? (N.F.U.)

2. Describe any rough, prickly, or hairy seeds or fruits, explaining the form and nature of the outgrowths and their use. (N.F.U.)

3. What wild fruits and wild flowers would you expect to find in September in your part of the country? In what kinds of places would you look for them? (N.F.U.)

4. Describe and draw the fruit of a field geranium, and point out the uses of some of its peculiarities. (1898)

5. Describe the seed vessels of a pansy. Draw one entire, and also burst open. How does it scatter the seeds? (1898)

6. Shortly describe the fruit of an apple or pear, and of a cherry or plum. Point out the chief differences between them. (1901)

7. How are the seeds of cherry, field geranium, and pine or birch dispersed? (1901)

8. Give examples of seeds which are dispersed by the aid of birds or other animals, explaining in each case how the dispersal is effected. (1893)

9. Why is it an advantage to some fruits to be (_a_) brightly coloured, (_b_) sweet? Give examples.

10. A school museum contains, among other things, some dandelion fluff, a dish of marrowfat peas, a few nodules of garlic, and some hawthorn berries. How could you employ these to illustrate a lesson on plant germination? (Certificate 1903)