Tomato plants posses the same basic anatomy characteristics of all vascular plants. If you understand how a tomato stem grows, you will have a better knowledge of where and when to prune. Seventy five percent of tomato plants have sprawling stems, which continue to grow and produce flowers until frost. Pruning tomatoes in the correct places forces the plant into fruiting, and creates a stronger plant.
Types of Stem Growth
Tomato plant stems grow in two ways. Primary growth occurs when the plant sends out lateral stems at the end of the shoot. At some point, older shoots stop growing longer, become woody, and take on the role of supporting the plant. However, the tomato continues to grow, so it has to transport more and more nutrients and water. Woody tomato stems then undergo secondary growth, when existing cells change roles and form new vascular tissue. Older vascular tissues then become support tissues.
Cell and Tissue Types
Tomato stem cells form tissues with differing functions. The outer tissue, or epidermis, houses "stomata" -- tiny openings that allow carbon dioxide, oxygen and water to move in and out of the plant. The cortex resides just inside the epidermis. The vascular tissues of a tomato plant are separated by the vascular cambium, which continuously builds new tissues of two types during secondary growth. The innermost, spongy core of the stem is called pith and serves primarily as a storage area.
The tissues that transport water and nutrients to and from the leaves and roots are located on either side of the vascular cambium. The phloem tissue is positioned just inside the cortex. Similar to the arteries in a human body, phloem tubes are responsible for transporting the food created during photosynthesis down to the roots. The xylem tissue is located between the cambium and pith. Xylem tubes transport water from the roots back to the leaves and stem tips.
When terminal buds at stem tips are active, the stem grows longer. At some point a bud stops growing, often when it begins to form flowers. Lateral stems then begin to lengthen.The first or main stems of tomato plants stop elongating relatively early in a tomato plant's life, compared with other vascular plants.
Tomatoes also form adventitious roots (roots growing on stems) on older wood. If the plant falls over or the stem otherwise comes in contact with the soil, these tiny plant parts emerge from the stem and grow into roots.