When it gets there it finds our old friends the sunbeams hard at work. If you have ever tried to grow a plant in a cellar, you will know that in the dark its leaves remain white and sickly. It is only in the sunlight that a beautiful delicate green tint is given to them, and you will remember from Lecture II. that this green tint shows that the leaf has used all the sun-waves except those which make you see green ; but why should it do this only when it has grown up in the sunshine }

The reason is this : when the sunbeam darts into the leaf and sets all its particles quivering, it divides the protoplasm into two kinds, collected into different cells. One of these remains white, but the other kind, near the surface, is altered by the sunlight and by the help of the iron brought in by the water. This particular kind of protoplasm, which is called "chloro phyll," will have nothing to do with the green waves and throws them back, so that every little grain of this protoplasm looks green and gives the leaf its green colour.

It is these little green cells that by the help of the sun-waves digest the food of the plant and turn the water and gases into useful sap and juices. We saw in Lecture III. that when we breathe-in air, we use up the oxygen in it and send back out of our mouths carbonic acid, which is a gas made of oxygen and carbon.

Now, every living thing wants carbon to feed upon, but plants cannot take it in by itself, because carbon is solid (the blacklead in your pencils is pure carbon), and a plant cannot eat, it can only drink-in fluids and gases. Here the little green cells help it out of its difficulty. They take in or absorb out of the air the carbonic acid gas which we have given out of our mouths, and then by the help of the sun-waves they tear the carbon and oxygen apart. Most of the oxygen they throw back into the air for us to use, but the carbon they keep. If you will take some fresh laurel-leaves and put them into a tumbler of water turned upsidedown in a saucer of

Oxygen-bubbles rising from laurel- water, and set the leaves in water. tumbler in the Sunshine, you will soon see little bright bubbles rising up \xxd clinging to the glass. These are bubbles of

oxygen gas, and they tell you that they have been set free by the green cells which have torn from them the carbon of the carbonic acid in the water.

But what becomes of the carbon ? And what use is made of the water which we have kept waiting all this time in the leaves ? Water, you already know, is made of hydrogen and oxygen ; but perhaps you will be surprised when I tell you that starch, sugar, and oil, which w^e get from plants, are nothing more than hydrogen and oxygen in different quantities joined to carbon.

It is very difficult at first to picture such a black thing as carbon making part of delicate leaves and beautiful flowers, and still more of pure white sugar. But we can make an experiment by which we can draw the hydrogen and oxygen out of common loaf sugar, and then you will see the carbon stand out in all its blackness. I have here a plate with a heap of white sugar in it. I pour upon it first some hot water to melt and warm it, and then some strong sulphuric acid. Tli'is acid does nothing more than simply draw the hydrogen and oxygen out. See ! in Carbon rising up from white sugar a few moments a black mass of carbon begins to rise, all of which has come out of the white sugar you saw just now.* You see,

The common dilute sulphuric acid of commerce is not strong enough for this experiment, and any child who wants to get pure sulphuric Acid must take some elder person with him, otherwise the chemist will not sell it to him. Great care must be taken in using it, as it burns everything it touches.

then, that from the whitest substance in plants we can get this black carbon ; and in truth, one-half of the dry part of every plant is composed of it.

Now look at my plant again, and tell me if we have not already found a curious history ? Fancy that you see the water creeping in at the roots, oozing up from cell to cell till it reaches the leaves, and there meeting the carbon which has just come out of the air, and being worked up with it by the sun-waves into starch, or sugar, or oils.

But meanwhile, how is new protoplasm to be formed "i for without this active substance none of the work can go on. Here comes into use a lazy gas we spoke of in Lecture III. There we thought that nitrogen was of no use except to float oxygen in the air, but here we shall find it very useful. So far as we know, plants cannot take up nitrogen out of the air, but they can get it out of the ammonia which the water brings in at their roots.

Ammonia, you will remember, is a strong-smelling gas, made of hydrogen and nitrogen, and which is often almost stifling near a manure-heap. When you manure a plant you help it to get this ammonia, but at any time it gets some from the soil and also from the rain-drops which bring it down in the air. Out of this ammonia the plant takes the nitrogen and

works it up with the three elements, carbon, oxygen, and hydrogen, to make the substances called albumin noids, which form a large part of the food of the plant, and it is these albuminoids which go to make

protoplasm. You will notice that while the starch

and oXh^r substances are only made of three elements.

che active protoplasm is made of these three added to a fourth, nitrogen, and it also contains phosphorus and sulphur.

And so hour after hour and day after day our primrose goes on pumping up water and ammonia from its roots to its leaves, drinking in carbonic acid from the air, and using the sun-waves to work them all up into food to be sent to all parts of its body. In this way these leaves act, you see, as the stomach of the plant, and digest its food.

Sometimes more water is drawn up into the leaves than can be used, and then the leaf opens thousands of little mouths in the skin of its under surface, which let the drops out just as drops of perspiration ooze throu2fh our skin when we are overheated. These little mouths, which are called stomates {a, Fig. 42) are made of two flattened cells, fitting against each other. When the air is damp and the plant has too much water these lie open and let it out, but when the air is dry, and Stomates c-. of a ^ leaf. c the plant wants to keep as much water as it can, then they are closely shut. There are as many as a hundred thousand of these mouths under one apple-leaf, so you may imagine how small ihey often are.

Plants which only live one year, such as migno« nette, the sweet pea, and the poppy, take in just enough food to supply their daily wants and to make the seeds we shall speak of presently. Then, as soon as their seeds are ripe their roots begin to shrivel, and

water is no longer carried up. The green cells can no longer get food to digest, and they themselves are broken up by the sunbeams and turn yellow, and the plant dies.

But many plants are more industrious than the stock and mignonette, and lay by store for another year, and our primrose is one of these. Look at this thick solid mass below the primrose leaves, out of which the roots spring.* This is really the stem of the primrose hidden underground, and all the starch, albuminoids, &c., which the plant can spare as it grows, are sent down into this underground stem and stored up there, to lie quietly in the ground through the long winter, and then when the warm spring comes this stem begins to send out leaves for a new plant.

We have now seen how a plant springs up, feeds itself, grows, stores up food, withers, and dies ; but we have said nothing yet about its beautiful flowers or how it forms its seeds. If we look down close to the bottom of the leaves in a primrose root in spring-time, we shall always find three or four little green buds nestling in among the leaves, and day by day we may see the stalk of these buds lengthening till they reach up into the open sunshine, and then the flower opens and shows its beautiful pale-yellow crown.

We all know that seeds are formed in the flower, and that the seeds are necessary to grow into new plants. But do we know the history of how they are formed, or what is the use of the different parts of the bud t Let us examine them all, and then I

• See the plant in the foreground of the heading of the lecture.

think you will agree with me that this is not the least wonderful part of the plant.

Remember that the seed is the one important thing, and then notice how the flower protects it. First, look at the outside green covering, which we call the calyx. See how closely it fits in the bud, so that no insects can creep in to gnaw the flower, nor any harm come to it from cold or blight. Then, when the calyx opens, notice that the yellow leaves which form the crown or corolla, are each alternate with one of the calyx leaves, so that anything which got past the first covering would be stopped by the second. Lastly, when the delicate corolla has opened out, look at those curious yellow bags just at the top of the tube {b^2^ Fig. 43). What is their use ? Fig. 43.

The two forms of the Primrose-flower.

fz, Stigma or sticky head of the seed-vessel, Anthers of the ^tannens. Corolla or crown of the flower, Calyx or outer covering, r^', Seed-vessel. A, Enlarged pistil, with pollen-grain resting on the stigma and growing down to the ovule, Ovules.

But I fancy I see two or three little questioning faces which seem to say, " I see no yellow bags at

the top of the tube." Well, I cannot tell whether you can or not in the specimen you have in your hand ; for one of the most curious things about primrose flowers is, that some of them have these yellow bags at the top of the tube and some of them hidden down right in the middle. But this I can tell you : those of you who have got no yellow bags at the top will have a round knob there (l ^, Fig. 43), and will find the yellow bags {U) buried in the tube. Those, on the other hand, who have the yellow bags (2 by Fig. 43) at the top will find the knob {a) half-way down the tube.

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