In forest trees, the central pith becomes almost obliterated and the old stem practically consists of wood, bast, and bark. The various annual rings, and the bast, of a woody stem are joined together by a number of radiating horizontal spokes called medullary rays ([Fig. 43], ms, ms′, ms″, ms‴). These conduct water and food materials across the stem from layer to layer. The beautiful lines and patches called “silver grain,” which may be seen in oak furniture, consist of the medullary rays exposed in radial-longitudinal section.
Bark.—The very young stem is surrounded by a thin waterproof skin, perforated by stomata ([p. 53]) as the skin of a leaf is; and the part immediately below the skin possesses leaf-green, and can therefore decompose carbon dioxide as a leaf does ([p. 51]). An old stem, on the other hand, is covered by a layer of tough bark (br, [Fig. 43]), which splits from time to time, owing to the stretching which is caused by the increasing thickness of the wood. The bark begins as a layer of cork, which forms on the outer side of the bast. The cork cuts off the food supply from all the external tissues, which die. Bark, therefore, consists of the cork and the dead layers outside it.
Grass stems.—The great strength of grass stems—so apparent when we try to bend a straw—is largely due to silica, the substance of which rock-crystal and ordinary sand are composed. When a straw is burnt, this remains as a hollow cylinder of mineral matter. The great strength of the cylindrical form has already been referred to. It is very well seen also in a bamboo stem, and anyone who has blunted the edge of his knife on a piece of bamboo will appreciate the additional hardness which is given by the presence of mineral matter.
13. CLIMBING STEMS.
1. Hooking stems.—Examine a bramble or a wild rose plant in a hedge. Why does it need to climb? How does it climb? Pull a branch and notice by what means it resists the pull. What is the shape of the prickles? Do they point upwards or downwards? On what parts of the plant are the prickles found? Notice how easily a prickle may be pushed off, sideways. Is it as easy to tear it off lengthways? Is the prickle a little branch, or merely an extension of the rind?
Contrast a prickle with the thorn of the hawthorn. The thorn does not come off easily, and it contains a woody core which is continuous with the wood of the branch. Cut lengthwise through the thorn and the branch which bears it to see this. The thorn is a short pointed branch; it arises in the axil of a leaf, and sometimes bears leaves itself.
2. The ivy.—Observe how the climbing stem of the ivy is attached to a wall or tree. It puts out a line of roots on its shaded side. These roots give out a sticky fluid, which, on hardening, fixes them to the wall.
3. Twining stems.—Watch the growth of a convolvulus seedling. At first the young stem grows straight up, but soon the tip begins to move round and round. Try to find out how long it takes to describe one revolution. Put a long stick in the ground near the plant and notice how, when the revolving stem touches the stick, the spiral is henceforth described round the support and the stem consequently clings to it. Lay your watch face-upward, and notice whether the stem moves in the same direction as the hands (the “clockwise” direction), or in the opposite (“counter-clockwise”) direction.
Make similar observations on the hop, honeysuckle, and scarlet runner, and note the results.
4. Leaf climbers.—Examine a climbing tropœolum (often, though wrongly, called “nasturtium”). Which parts of the plant clasp the support? Watch a young plant coming up. Does the stem revolve before the leaf stalks come in contact with the support? Compare the clematis.