As well as supporting the leaves, the stems have another very important duty, something like that of the roots. Just as the roots absorb the water from the soil and carry it up, passing it on to the stems, so the stems carry it on to the place where it is finally used, that is, to the leaves. In both stems and roots there are channels or “water-pipes” which carry water about, as well as other special “pipes” which carry the manufactured food.
So that the two chief duties of stems are to act as supports for the leaves and flowers, and to carry the food materials and water between the roots and leaves.
Just as we found in the case of the roots, there are many extra duties which the stems may take over, and as a result, we find great variety in the appearance of stems. For example, in some plants the stem does not grow up into the air at all, but creeps along just below the surface of the ground. This you may see if you dig up a Solomon’s Seal or an iris, when you will find that the stem looks very like a thick root running horizontally in the ground. That it is really a stem you can tell from the fact that the leaves grow out from it, and you can see the scars of old ones as well as the present leaves, and also some little brown scaly leaves, and a large number of adventitious roots. The stem is rather swollen with food materials which are stored up in it, and it is not coloured green like many of those growing in the air. Such a stem, creeping under the earth, and only sending its green leaves into the air, has a special name, and is called a Rhizome. Many plants have such stems, particularly ferns, as you can see very well if you dig up a bracken.
Fig. 46. Underground stem of the Solomon’s Seal, called a Rhizome. It has many scaly leaves, s and a shoot A which will come out into the air bearing green leaves. B is the scar left by the similar shoot of last year. r are the adventitious roots which come out all over the stem.
Fig. 47. A Potato: s, the stem attaching it to the main stem; e, scale-leaf; b, bud in its axil; t, tip of the Potato with several buds, some of which are sprouting.
Some of the underground stems which store food are still more modified, so that it is very hard indeed to tell what they really are. This is the case in the potato, which you would naturally think at first is a swollen root, like those we saw in the dahlia (fig. 39). That it is really a stem you can see by examining the “eyes” carefully. The eyes (see fig. 47) are buds with scale leaves round them, and at the tip of the potato we can see several such buds together (fig. 47 t). The whole potato is a very much swollen stem which is packed with food and has all its other parts so reduced that it is difficult to recognise them. Such special stems are called Tubers.