Outlines of Lessons in Botany Part 4

Begin by telling the pupils to prepare for their first lesson a description of the roots of their seedlings. Those grown on sponge or paper will show the development of the root-hairs, while those grown on sand are better for studying the form of the root. Give them also some fleshy root to describe, as a carrot, or a radish; and a spray of English Ivy, as an example of aerial roots.

Throughout these lessons, the method is pursued of giving pupils specimens to observe and describe before teaching them botanical terms. It is better for them to name the things they see than to find examples for terms already learned. In the first case, they feel the difficulty of expressing themselves and are glad to have the want of exact terms supplied. This method is discouraging at first, especially to the younger ones; but, with time and patience, they will gradually become accustomed to describe whatever they can see. They have, at any rate, used their eyes; and, though they may not understand the real meaning of anything they have seen, they are prepared to discuss the subject intelligently when they come together in the cla.s.s. If they will first write out their una.s.sisted impressions and, subsequently, an account of the same thing after they have had a recitation upon it, they will be sure to gain something in the power of observation and clear expression. It cannot be too strongly urged that the number of facts that the children may learn is not of the slightest consequence, but that the teacher should aim to cultivate the quick eye, the ready hand, and the clear reason.

The root of the Morning-Glory is _primary_; it is a direct downward growth from the tip of the caulicle. It is about as thick as the stem, tapers towards the end, and has short and fibrous branches. In some plants the root keeps on growing and makes a _tap-root_; in the Bean, it soon becomes lost in the branches. These are all simple, that is, there is but one primary root. Sometimes there are several or many, and the root is then said to be _multiple_. The Pumpkin is an example of this. The root of the Pea is described in the older editions of Gray's Lessons as being multiple, but it is generally simple. Indian Corn, also, usually starts with a single root, but this does not make a tap-root, and is soon followed by many others from any part of the caulicle, or even from the stem above, giving it the appearance of having a multiple root.

The root of the Radish is different from any of these; it is _fleshy_.

Often, it tapers suddenly at the bottom into a root like that of the Morning-Glory with some fibres upon it. It is, in fact, as the Morning-Glory would be if the main root were to be thickened up by food being stored in it. It is a primary tap-root. The radish is _spindle-shaped_, tapering at top and bottom, the carrot is _conical_, the turnip is called _napiform_; some radishes are shaped like the turnip.

The aerial roots of the English Ivy answer another purpose than that of giving nourishment to the plant. They are used to support it in climbing.

These are an example of _secondary_ roots, which are roots springing laterally from any part of the stem. The Sweet Potato has both fleshy and fibrous roots and forms secondary roots of both kinds every year.[1] Some of the seedlings will probably show the root-hairs to the naked eye. These will be noticed hereafter.

[Footnote 1: Gray's Lessons, p. 35, Fig. 86.]

[Ill.u.s.tration: FIG. 11.--1. Tap-root. 2. Multiple root of Pumpkin. 3.

Napiform root of Turnip. 4. Spindle-shaped root of Radish. 5. Conical root of Carrot. 6. Aerial roots of Ivy.]

It is my experience that pupils always like cla.s.sifying things under different heads, and it is a good exercise. The following table may be made of the roots they have studied, adding other examples. Dr. Gray says that ordinary roots may be roughly cla.s.sed into fibrous and fleshy.[1]

Thome cla.s.ses them as woody and fleshy.[2]

[Footnote 1: Gray's Lessons, p. 34.]

[Footnote 2: Text-book of Structural and Physiological Botany. Otto Thome.

Translated and edited by Alfred W. Bennett, New York. John Wiley and Sons.

1877. Page 75.]

ROOTS.

------------------------------------------ _Primary_. _Secondary_.

-------------------------------- _Fibrous_. _Fleshy_. Roots of cuttings Aerial roots.

------------------- Sweet potatoes.[3]

_Simple_. _Multiple_. _Simple_.

Morning Glory. Pumpkin Carrot.

Sunflower. Radish.

Pea. Turnip.

Bean. Beet.

Corn. Corn.

[Footnote 3: The Irish potato will very likely be mentioned as an example of a fleshy root. The teacher can say that this will be explained later.]

2. _Fleshy Roots_.--The scholars are already familiar with the storing of food for the seedling in or around the cotyledons, and will readily understand that these roots are storehouses of food for the plant. The Turnip, Carrot, and Beet are _biennials_; that is, their growth is continued through two seasons. In the first year, they make a vigorous growth of leaves alone, and the surplus food is carried to the root in the form of a syrup, and there stored, having been changed into starch, or something very similar. At the end of the first season, the root is filled with food, prepared for the next year, so that the plant can live on its reserve fund and devote its whole attention to flowering. These roots are often good food for animals. There are some plants that store their surplus food in their roots year after year, using up in each season the store of the former one, and forming new roots continually. The Sweet Potato is an example of this cla.s.s. These are _perennials_. The food in perennials, however, is usually stored in stems, rather than in roots, as in trees. _Annuals_ are generally fibrous-rooted, and the plant dies after its first year. The following experiment will serve as an ill.u.s.tration of the way in which the food stored in fleshy roots is utilized for growth.

Cut off the tapering end of a carrot and scoop out the inside of the larger half in the form of a vase, leaving about half of the flesh behind.

Put strings through the upper rim, fill the carrot cup with water, and hang it up in a sunny window. Keep it constantly full of water. The leaf-buds below will put forth, and grow into leafy shoots, which, turning upwards, soon hide the vase in a green circle. This is because the dry, starchy food stored in the carrot becomes soft and soluble, and the supply of proper food and the warmth of the room make the leaf-buds able to grow.

It is also a pretty ill.u.s.tration of the way in which stems always grow upward, even though there is enough light and air for them to grow straight downwards. Why this is so, we do not know.

3. _Differences between the Stem and the Root.--_Ask the pupils to tell what differences they have found.

_Stems_. _Roots_.

Ascend into the air. Descend into the ground.

Grow by a succession of similar Grow only from a point parts, each part when young just behind the tip.

elongating throughout.

Bear organs. Bear no organs.

There are certain exceptions to the statement that roots descend into the ground; such as aerial roots and parasitic roots. The aerial roots of the Ivy have been mentioned. Other examples of roots used for climbing are the Trumpet Creeper _(Tecoma radicans)_, and the Poison Ivy _(Rhus Toxicodendron)_. Parasitic roots take their food ready-made from the plants into which they strike. The roots of air-plants, such as certain orchids, draw their nourishment from the air.

The experiment of marking roots and stem has been already tried, but it should be repeated. Repet.i.tion of experiments is always desirable, as it fixes his conclusions in the pupil's mind. The stem grows by a succession of similar parts, _phytomera_, each part, or _phyton_, consisting of node, internode, and leaf. Thus it follows that stems must bear leaves. The marked stems of seedlings show greater growth towards the top of the growing phyton. It is only young stems that elongate throughout. The older parts of a phyton grow little, and when the internode has attained a certain length, variable for different stems and different conditions, it does not elongate at all.

The root, on the contrary, grows only from a point just behind the tip.

The extreme tip consists of a sort of cap of hard tissue, called the root-cap. Through a simple lens, or sometimes with the naked eye, it can be distinguished in most of the roots of the seedlings, looking like a transparent tip. "The root, whatever its origin in any case may be, grows in length only in one way; namely, at a point just behind its very tip. This growing point is usually protected by a peculiar cap, which insinuates its way through the crevices of the soil. If roots should grow as stems escaping from the bud-state do,--that is, throughout their whole length--they would speedily become distorted. But, since they grow at the protected tips, they can make their way through the interstices of soil, which from its compactness would otherwise forbid their progress."[1]

[Footnote 1: Concerning a few Common Plants, p. 25.]

The third difference is that, while the stem bears leaves, and has buds normally developed in their axils, roots bear no organs. The stem, however, especially when wounded, may produce buds anywhere from the surface of the bark, and these buds are called _advent.i.tious_ buds. In the same manner, roots occasionally produce buds, which grow up into leafy shoots, as in the Apple and Poplar.[1]

[Footnote 1: See Gray's Structural Botany, p. 29.]

It should be made perfectly clear that the stem is the axis of the plant, that is, it bears all the other organs. Roots grow from stems, not steins from roots, except in certain cases, like that of the Poplar mentioned above. This was seen in the study of the seedling. The embryo consisted of stem and leaves, and the roots were produced from the stem as the seedling grew.

For ill.u.s.tration of this point, the careful watching of the cuttings placed in water will be very instructive. After a few days, small, hard lumps begin to appear under the skin of the stem of the broken seedling Bean. These gradually increase in size until, finally, they rupture the skin and appear as rootlets. Roots are always thus formed under the outer tissues of the stem from which they spring, or the root from which they branch. In the Bean, the roots are in four long rows, quartering the stem.

This is because they are formed in front of the woody bundles of the stem, which in the seedling Bean are four. In the Sunflower the roots divide the circ.u.mference into six parts. In some of my cuttings of Beans, the stem cracked in four long lines before the roots had really formed, showing the parenchyma in small hillocks, so to speak. In these the gradual formation of the root-cap could be watched throughout, with merely a small lens. I do not know a better way to impress the nature of the root on the pupil's mind. These forming roots might also be marked very early, and so be shown to carry onward their root-cap on the growing-point.

4. _Root-hairs_. These are outgrowths of the epidermis, or skin of the root, and increase its absorbing power. In most plants they cannot be seen without the aid of a microscope. Indian Corn and Oats, however, show them very beautifully, and the scholars have already noticed them in their seedlings. They are best seen in the seedlings grown on damp sponge. In those grown in sand, they become so firmly united to the particles of soil, that they cannot be separated, without tearing the hairs away from the plant. This will suggest the reason why plants suffer so much from careless transplanting.

The root-hairs have the power of dissolving mineral matters in the soil by the action of an acid which they give out. They then absorb these solutions for the nourishment of the plant. The acid given out was first thought to be carbonic acid, but now it is supposed by some experimenters to be acetic acid, by others to vary according to the plant and the time.

The action can be shown by the following experiment, suggested by Sachs.

[Ill.u.s.tration: Fig. 12. I. Seedling of _Sinapis alba_ showing root-hairs.

II. Same, showing how fine particles of sand cling to the root-hairs.

(Sachs.)]

Cover a piece of polished marble with moist sawdust, and plant some seeds upon it. When the seedlings are somewhat grown, remove the sawdust, and the rootlets will be found to have left their autographs behind. Wherever the roots, with their root-hairs have crept, they have eaten into the marble and left it corroded. The marks will become more distinct if the marble is rubbed with a little vermilion.

In order that the processes of solution and absorption may take place, it is necessary that free oxygen should be present. All living things must have oxygen to breathe, and this gas is as needful for the germination of seeds, and the action of roots and leaves, as it is for our maintenance of life. It is hurtful for plants to be kept with too much water about their roots, because this keeps out the air. This is the reason why house-plants are injured if they are kept too wet.

A secondary office of root-hairs is to aid the roots of seedlings to enter the ground, as we have before noticed.

The root-hairs are found only on the young parts of roots. As a root grows older the root-hairs die, and it becomes of no further use for absorption.

But it is needed now for another purpose, as the support of the growing plant. In trees, the old roots grow from year to year like stems, and become large and strong. The extent of the roots corresponds in a general way to that of the branches, and, as the absorbing parts are the young rootlets, the rain that drops from the leafy roof falls just where it is needed by the delicate fibrils in the earth below.[1]

[Footnote 1: Reader in Botany. VI. The Relative Positions of Leaves and Rootlets.]

5. _Comparison of a Carrot, an Onion, and a Potato_.--It is a good exercise for a cla.s.s to take a potato, an onion, and a carrot or radish to compare, writing out the result of their observations.