The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Fig. 4. A, outside of Bean; (h) black scar showing where the bean was attached to the pod; (r) ridge made by young root; B, bean split open; (n) nurse leaves; (p) baby plant; (a) scar where the baby plant was separated from the nurse leaf on that side.
In the case of maize and corn, however, you will find that the seed does not split into two equal parts like the bean, but that the young plant lies at one side of the seed, and a solid white mass fills the rest of the space (see fig. 5). There are also differences in the seedlings which you will notice when they begin to grow.
Fig. 5. A, outside of Maize fruit, showing the embryo (e) on one side; B, sprouting plant, showing the root (r) and shoot (s); C, the same further grown.
Now that you have examined some seeds, you should start a number growing, so as to have plenty to watch. They will grow more quickly if you soak them in water for a night before you plant them in damp sawdust, and keep them moist and fairly warm all the time. You should have a number of seeds of each kind planted together to provide enough for you to dig up one of them every day and examine it fully, inside as well as out. Make a drawing of each one so that you will have a complete series of drawings showing how the young plants grow. This will kill them, so that you must leave at least one seedling which is never touched, and which you can watch all through its life.
Fig. 6. Growth of Bean seedling: A, the root only showing; B, the root lengthening and shoot appearing.
As the young plant grows, notice how it breaks away from the protection of its nurse leaves; first the root comes out and bends downwards into the sawdust (see fig. 6 A), then the little shoot which bends up into the air.
Whichever way you plant the seeds you will find this is always the case, for even if you start with the root pointing up, it will bend round and grow downwards while the shoot bends up (see p. 41).
As the plant gets bigger, side roots grow out from the main one, and the little leaves of the shoot begin to open out—the whole plant is growing (see fig. 7).
Fig. 7. Later stage in the growth of Bean seedling; side roots developed, and the shoot enlarged.
Now we may perhaps begin to find out something about the question of feeding in plants. What are the nurse-leaves doing all the time the plant is growing? You will find in the bean that the seed coats may split open a little, but that on the whole the cotyledons remain all the time enclosed in them, and attached to the young shoot (see fig. 7). Examine the nurse leaves of seedlings of different ages, and you will see that they are much less thick and fleshy in the older seedlings. As the plant gets bigger the nurse-leaves get thinner and less until they become merely dry shrivelled remnants.
Now, what use could the cotyledons be if they only shrivel away?
Take a freshly soaked seed and cut a thin slice of the nurse-leaves and drop it into a little solution of iodine;[1] the tissue will go a violet blue colour. Then drop iodine on a piece of bread, a piece of potato, and some boiled rice, and you will find that they also go blue, or almost black. The food in the nurse-leaves is in some ways the same as that in bread, potato, and rice, and in many other things we eat.
The part of the food which goes blue with iodine is starch, and this blue colouring of starch with iodine is an easy and safe test for it. You will see the same colour if you take some ordinary laundry starch and stir it up with hot water and a little iodine. Look now at the corn seed; the white solid mass in the seed contains starch, as you can prove with iodine, and although it is not in the cotyledon, yet it is quite near the young plant, which can get at it easily.
We have found, therefore, that young plants have a store of food in their nurse-leaves, or near them in the seed, and that this food is the same as very much of our own food, that is, it is starch. There are other food substances present, too, but they are more difficult to find. The seed, therefore, contains not only the young living plant itself, but also a storehouse of food for its use, and as the plant grows we see this store getting less and less in the shrivelling cotyledons. This shows that the young plant uses up this food in the course of its growth.
But you must not forget that, although we find the young plants provided with food in this way, we have not yet settled the question of the food supply for all plants. As we see, the cotyledons shrivel up and are emptied of their store long before the plant is full grown. Remember that baby calves have milk for food, while old cows have grass. And when the store of food supplied in the seed is finished the older plants must find new supplies for themselves.
In growing seedlings you must always keep them well supplied with water, the soil or sawdust in which they grow must be kept moist. If you take one out of the sawdust and try to grow it only in the air, you will find that it soon dies. Even for the seedling the storehouse of food is not enough; it requires to have water too.
You can keep seedlings growing quite well, however, if you place them in glass jars so that their roots are in water, or even in closed glass jars standing over water, so that the air is thoroughly moist. You will then be able to see very well numbers of fine white hairs on the roots (see fig. 8). These hairs are very important and absorb the water which keeps the whole plant moist.
Fig. 8. Maize seedlings growing enclosed in damp air, supported on a wire stand over dish of water so that their roots do not touch it, but grow in the air. Notice the “root hairs” growing out from the roots.
You have now seen that seedlings require water for their life just as animals do; and also that young plants are provided by their parents with a store of food which is largely starch, and which they use up during their early growth.
FOOD MATERIALS OF THE OLDER PLANT
(1) IN THE SOIL
As we have just seen, young seedlings are supplied with stores of food, starch, and other things, which are packed in their cotyledons and are used up by them as they grow. But we also saw that as the plant gets older these stores get emptier, and finally the nurse leaves shrivel up entirely when their contents are exhausted. All the same, however, the plant continues to grow. Surely it cannot do this on nothing, any more than an animal could? When the young calves cease to be fed with milk, their food changes, and they begin to eat grass; this gives them individually more work, for grass is not a “prepared food” like milk. Very much the same thing happens with seedlings. Their prepared food supply gets used up, and they must find food for themselves. Where do they find it?
When you remember the fine hairs on the young parts of roots which absorb water from the soil or sawdust, it is quite natural to think at once of the soil as a possible place for them to find their food; and, indeed, this is partly the case. The water in the soil is not perfectly pure, for there are many different “salts” dissolved in it. By “salts” one does not mean only table salt, but also any kind of mineral in solution, such as salts of iron or portions of chalk or limestone, or even some of the minerals which make up granite. These may all be dissolved in rain-water just as sugar is dissolved in your tea, and so spread equally through it. As the water enters the roots of plants through the hairs, these dissolved salts come in with it, and so get distributed over the whole plant. The root hairs cannot “eat” particles of soil, but they twine in among the fine grains and absorb the little films of water which cling to them.
Fig. 9. Root hairs growing among soil particles.
(Much magnified.)
You can find out some of the importance of these mineral salts in the life of the plant, if you do the following experiment.
Take several seedlings which have already grown enough to have nearly exhausted the supply of food in their cotyledons. These you must grow in jars of pure distilled water, to which you have added certain salts which have been found to be the important ones in the soil water and plant food. By giving the plant nothing but these salts and distilled water you know just what it gets. Distilled water is made by catching and condensing steam, and it has no salts dissolved in it; while ordinary tap water has run off some mountain side or risen in some spring from the rocks, and it has many salts in it already, so that it is useless for this experiment.
Take three big glass jars, each with one litre of distilled water, and label them A, B, and C. Into A put nothing further, into B put the following salts, which have been weighed out carefully either by you or by a chemist:—
then add to C all these salts, and also one or two drops of a dilute solution of iron chloride.
Into the jars fit corks which are split, with a hole in the centre, and pack a plant into each with the part of the stem between the corks wrapped round with cotton wool (see fig. 10), and so fix the plant that its roots are in the solution and its stem and leaves in the air
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