CHAPTER V.
HOW STEMS DO THEIR WORK
11. THE DUTIES OF STEMS.
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.