Birds and other animals are intended to scatter the fruits and seeds, and so the fruits must be easily distinguished at a distance. The seeds are taken to some other place, where they germinate and form a new plant. This furnishes the clue and guide to many other peculiarities in fruits and seeds.
The pleasant smell of ripe apples, plums, strawberries, and other fruits, also attracts birds and other animals. But the sugary juice and delicious flesh is developed entirely for the purpose of making it worth a bird's while to eat it. The amount of sugary matter is enormous, and the seeds seem very small and inconspicuous compared with this luscious mass. The sugar is produced very rapidly towards the end of the ripening period.
A Cucurbita fruit, for instance, may increase in weight at the rate of·0032 ounce per minute. All who have gathered strawberries know how quickly they ripen.
The way in which the sugar is formed is not understood, but unripe fruits contain bitter, unwholesome acids and essences which may produce colic or very unpleasant effects if the fruits are eaten green. Thus the colour is a guide to the animal, who is not supposed to eat the fruit until it is ripe; if eaten green, the seeds inside the fruit are quite destroyed and cannot germinate. Yet animals are so greedy that young birds, young animals of all sorts (even girls and boys) will and do eat green or half-ripe fruit. In this present year there is no doubt that many children have suffered for having done this. Yet if we come to think of it, throughout all the millions of years during which fruits have ripened, Nature has every year clearly told young pterodactyls and other lizards, young birds, young monkeys, and young people to wait till the fruit is ripe. None of them have learnt to do so.
When investigating by experiment, on the vile body, the properties of plums, strawberries, and other fruits, you are sure to find here and there one that has decayed and become rotten. In most cases this is because a bird has pecked a hole in it, or because the outside skin has been broken by a wasp. The sugar has then begun to ferment. Why does it do so?
If you gather a few fruits, put them into a jar of sugar-water, and leave it after closing the mouth with a bunch of cotton wool, then in a day or two fermentation begins and alcohol is produced. That is because, on the outside of the fruit, there were hundreds of an objectionable little fungus. It lives upon sugar and turns the latter into alcohol. This yeast fungus is really a living distillery. It lives in the midst of alcohol all its life, dying eventually (like the Duke of Clarence in his butt of Malmsey wine) by alcoholic poisoning, which it has brought about by its own work. This little yeast fungus can only be seen with a microscope. From a rotten fruit it drops on to the ground, where it remains all winter. Next spring certain small insects (green-fly and the like) carry some of these yeasts from the earth to next year's fruits. But the skin of the plum or apple, or the hairs on a gooseberry, or the delicate, waxy bloom on a grape, will prevent these insects or wasps from laying open the sugar inside the fruit to the attacks of yeasts and other fermenting fungi.
Some fruits appear to have "favourites"; they seem to prefer that large animals should eat them. If you look carefully at a piece of orange peel, and cut a small piece across, you will see distinctly small resin pits full of a curious essence which gives the characteristic taste to marmalade. This bitter stuff will prevent wasps from touching the sugar. It is, however, a valuable material, and some kinds of lemons, etc., are grown chiefly for this oil, which is obtained by scraping the peel with a little saucer which is studded with short pins.
A still more extraordinary fruit is the prickly pear; this is very delicious though very difficult to eat. Indeed, only monkeys and man seem able to enjoy it. The sugary part and the seeds form a little round mass in the inside. The outside part, though also fleshy, contains hundreds of minute mineral needles, which stick in the tongue and lips and cause most painful inflammation. The monkey eats the prickly pear with very great caution, getting his fingers into the top and scooping out the sugary part. Man requires a teaspoon to do this satisfactorily.
Another very curious point about these fleshy fruits (and also ordinary ones) is the strength of the seed inside. It does not look very strong.
But an orange seed, for instance, will not be in the least injured if you put it between two glass plates and gradually press upon the upper one up to even a pressure of some thirty pounds. Even hemp seed, which seems quite weak, will endure a weight of four pounds. It is impossible to break a prune stone, or to injure a date stone, by standing with your whole weight upon it.