Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 19

1. The shapes of stems.—Cut across a deadnettle stem and a wallflower stem and examine the shape of the sections. The former is square, the latter is five-ribbed. Is there any relation between the form of the stem and the arrangement of the leaves?

2. The “bleeding” of stems.—Cut through the lower part of a scarlet-runner plant in spring. Can you see any water escaping from that part of the stem still in the ground? Similarly, cut back a sunflower stem when it is from ½ to 1 inch thick. Does the water exude from all parts of the cut surface equally, or does it come from definite channels? To see this better, dry the cut end with blotting paper and examine the surface with a lens. “Bleeding” is best seen in vine stems; if a young vine is available, cut it back in spring and observe the large escape of water.

3. The water-current travels along definite channels.—Colour some water with red ink and put in it the stalks of cut white flowers such as snowdrops or narcissi. The stalks and flowers soon become veined with red. Cut the stalks across and see that the strands in the interior are also coloured. The coloured water has evidently travelled along definite channels.

4. The food-channels in a soft stem.—(a) Take a stout piece of the stem of a deadnettle, including three nodes (p. 45), and slit it down in a line between the end pairs of leaves. Then boil the stem in water until the internal tissue is soft. Take the stem out and carefully scrape away all the soft material until the woody strands can be well seen. These are the food-channels. Notice their arrangement in the stem, and the manner in which they run out to the leaves.

(b) Similarly, examine the strands in a piece of sunflower stem, and in an old cabbage stalk.

5. The path of the water-current in a woody stem.—Take a leafy twig of elder or laurel and, from the part of the shoot below the leaves, remove a ring of about an inch of the bark and the soft tissues which lie beneath it, so as to expose the wood. Put the end of the twig (below the ring) in water. The leaves remain fresh and crisp, showing that the water travels either along the wood or the pith, or along both.

6. The water travels along the wood.—Put a similar leafy twig of elder dipping in water which has been coloured by red ink. Expose to sunlight, and after an hour or two cut open the twig and observe which parts are coloured. The bark and the soft tissues between it and the wood are not coloured. The wood is stained red. The pith is not coloured.

7. The path of the leaf-made food.—(a) Take a leafy branch of a tree—e.g. willow—and near the bottom remove a ring of the bark and the soft tissues lying between bark and wood. Put the twig in water, so that the ringed part and a few inches above shall be immersed. After a time roots are produced above the cut. If any arise from the stripped part they are few in number and much shorter than those above.

(b) Place a similar but uninjured twig in water, and notice that the new roots are produced at the end. What is the reason for the difference?

The duties of stems.—No one can study leaves without being impressed by the great importance to the plant of the work which they do. Even the casual observation that year after year thousands of fresh leaves make their appearance would indicate this. And when it is learned that throughout the day every leaf is busily engaged in decomposing carbon dioxide, and joining up the carbon with the water and mineral compounds which have come up from the root—thus forming sugars, starch and a host of other valuable substances—some general idea is obtained that the trunk and spreading boughs of a tree may, after all, be of minor importance, and may exist mainly to help the leaves to perform their duties as perfectly as possible.

This is the true view to take of the stem and its branches; their duties are (1) to bear the leaves and spread them out, so that these will receive as much sunshine and fresh air as possible; (2) to supply the leaves with the water and mineral substances which they require for their work; and (3) to receive from the leaves and distribute to the rest of the plant the food materials which the leaves have prepared.

A stem, together with the leaves and branches which it bears, is called a shoot.

It will be found that almost all the variations in the structure and habits of stems are connected with the arrangements of the leaves. For example, the weight of the mass of leaves borne by a forest tree is very great, and they offer a great resistance to the wind; the trunk and its branches, therefore, are correspondingly strong and stout. Again, when a stem is ribbed or ridged in any particular manner it will generally be found that the ridges have a definite relation to the leaves and to their points of insertion on the stem. This has been seen (p. 44) to be the case in the stem of the wallflower.

The tip of a stem or branch is occupied by a bud. As the tip elongates (Fig. 40), the outer leaves of the bud become separated by internodes, and new leaves continually arise around the growing point.

The path of the water in the stem.—It may easily be shown that water travels up the stem to the leaves. It is common knowledge that slightly withered leaves or flowers become fresh and crisp again if the twig or stalk is put in water. If the stalks of white flowers—say snowdrops or narcissi—are put into water coloured with red ink, the flowers become veined with red, and, if the stalks are cut into, red strands may be seen. This experiment not only shows that water has travelled up the stalk, but it also shows that the water has passed along definite channels. When a vine stem is cut back in the spring, the water wells out rapidly from the end of that part of the stalk which is still in the ground. This bleeding, as it is called, may also be seen, though to a smaller extent, in stems of sunflower, scarlet-runner, and other plants. By drying the cut surface with blotting paper, and then examining it with a lens, it may be seen that the water escapes from the ends of certain little tubes; and it is possible in a boiled or rotting stem (Expt. 11. 4) to follow the distribution of the strands containing these tubes. The strands run along the length of the stem, outside the pith. Cross strands connect the main ones, chiefly at the leaf-levels (nodes); and other strands run out into the leaves to form the veins.

The tubes which convey the water current are in that part of the strand which is nearest the pith, and they become woody. Thus, if the current year’s growth of, say, a horse chestnut or an elder twig is cut across, a thin ring of wood is seen surrounding the soft pith. Outside the wood are the softer tissues, surrounded by the bark. A ring of the bark and soft tissues beneath it may be entirely removed from a growing twig, leaving the wood exposed, but the leaves above remain crisp and fresh, showing that this treatment has not interfered with their water supply. The water, therefore, travels along either the wood or the pith. That the wood and not the pith conducts the water may be shown by putting a leafy twig in water coloured with red ink. Only the wood is stained.

The path of the food made by the leaves.—The plant food which the leaves make is drained off into the stem, and distributed to the parts where growth is taking place. This food travels in the soft tissue—called the bast—which lies below the bark. This fact can be shown indirectly by removing a ring of bast from the lower part of a branch, say of willow, and putting the branch into water. When at length the cutting puts out roots, these spring from the top of the ring. If any spring from the stripped part they are markedly smaller, and fewer in number. In an uninjured cutting, which is of course supplied with prepared food along all its length by the bast vessels, such roots spring from the cut end. The supply of leaf-made food can also be cut off by ligaturing a twig below the leaves, as by twisting a wire or cord tightly round it. In such a case growth usually ceases in the part of the twig below the strangled part, while the upper part of the twig, to which the leaf-made food is now restricted, grows much more luxuriantly than before. Gardeners often produce unusually fine fruits by ligaturing the lower parts of the twigs on which the fruits are ripening.