Like the stem of kohlrabi, the potato expands nearly as much in diameter as in length. The potato swells as sugars made in above-ground leaves are pumped into it and converted into starch for long term storage.
| The potato is another example of shoot that has become specialized for storage. Unlike kohlrabi, which forms its spherical storage stem above-ground and makes foliage leaves, the potato forms underground and makes only small, scale-like leaves.
Like the stem of kohlrabi, the potato expands nearly as much in diameter as in length. The potato swells as sugars made in above-ground leaves are pumped into it and converted into starch for long term storage. |
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As with stems of other kinds of plants, we can look along the length of the potato to find its nodes and internodes. The small, scale-like leaves have usually worn away before the potato is harvested; however, we can readily find the so-called "eyes" that are the nodes of the potato stem. Each "eye" has semicircular scar that marks the position where the small, scale-like leaf was borne. Just above the semicircular leaf scar is a small protrusion: this is an axillary bud.
Axillary buds are young, dormant branches. They are called "axillary" because they form in the angle between the leaf and the stem: this is the leaf "axil." |
When an axillary bud begins to elongate it forms a branch. Branches tend to replicate the basic structure of the shoot system in being composed of stem and leaf components. We can grow potatoes in a garden by cutting a single potato into fragments that each contains an "eye." When planted, the axillary bud of each of potato piece will begin to grow outward from the "eye" to form an entirely new plant that will have an above-ground shoot with stems and leaves and below-ground roots. That new plant will also have the ability to form underground new potatoes.
Although potatoes grow underground, they are not roots. Growing underground does not make a plant part a root, and, similarly, growing above ground does not make a particular plant part a shoot. As we discussed above, roots and shoots are defined by where they form during the development of the plant embryo. Plants usually produce shoots that grow above-ground, where their leaves are exposed to the sunlight that is needed for photosynthesis, and roots that grow below ground, where they pull water and extract nutrients from the soil. Some plants, like potatoes, are specialized to make shoots below ground (as well as above ground). Another plant that does this is taro. Although taro is not commonly cultivated in North America, it can be found in some grocery stores. Because taro has a stem that grows only underground, it is mistakenly described as a root on some grocery labels. The underground stem of taro makes leaves that extend above ground, where they conduct photosynthesis, forming sugars that are moved into the specialized, storage stem. In the underground stem of taro, these sugars are converted into starch for long term storage, just as they are in potatoes.
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