Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Organs of Plants
3.6. Stem Diversity. Stem Metamorphoses

Stem growth occurs due to the elongation of the apical bud and is termed apical growth. It is also characteristic of first-, second-, and higher-order shoots developing from vegetative axillary buds. In grasses, horsetails, and other plants, rapid stem growth results from The activity of intercalary meristem located at the Base of the internodes. This type of stem growth is known as intercalary growth.

Plant stems exhibit great diversity. According to their cross-sectional shape, they can be: cylindrical (grasses), multifaceted (Apiaceae), triquetrous (sedges), quadrangular (Lamiaceae), flattened (Opuntia), etc. Based on their spatial orientation, stems are classified as erect, growing vertically upwards without bending under the weight of their branches, leaves, flowers, and fruits. Stems that creep along the ground and form adventitious roots at nodes in contact with moist soil are called prostrate or creeping. Stems with shortened internodes are referred to as runners (cucumbers, pumpkins), while those with elongated internodes are called stolons (strawberries). Procumbent stems that climb by attaching to other plants are termed climbing (peas, bedstraw, ivy). Twining stems (bindweed, hops) develop The ability to coil around the stems of other plants or Supports. Plants with twining and climbing stems are called lianas.

Based on their life span and habit, plants are classified into trees, shrubs, subshrubs, and herbs. A tree is a perennial plant with a distinct main stem that outgrows other shoots in length and thickness, forming a crown. In a shrub, the main stem is not prominently distinguished, and multiple lateral shoots develop near the soil surface. Subshrubs are plants with lignified or corky lower stem parts that remain perennial, while only the upper annual shoots die back. Herbs are plants whose aboveground parts die off annually at the end of the growing season. Among them are annuals, which complete their full vegetative and reproductive cycle (both aboveground and underground Organs) within one season, and biennials, which form underground organs in the first year and develop aboveground structures, completing their life cycle with fruit and seed production In the second year (carrots, cabbage). Perennials preserve their underground organs and are capable of annually producing renewal buds over many years (couch grass, sedges).

During their development, the stems of many plants undergo various Anatomical and morphological modifications (metamorphoses), which can be underground or aboveground.

Underground modifications include rhizomes (couch grass, lily of the valley), bulbs (onions, tulips), and tubers (potatoes, Jerusalem artichokes).

A rhizome is a modified underground SHOOT that possesses an apical bud, bears reduced leaves (scales) at the nodes, and produces adventitious roots. The rhizome serves not only as a storage organ for nutrients but also as a means of vegetative propagation and perennial Plant Regeneration.

Tubers are thickened, fleshy stem portions formed by one or several highly shortened internodes. Buds (eyes) are located On the surface of tubers, from which new shoots develop under favorable conditions.

A bulb is an underground shoot with shortened internodes and modified leaves. The basal plate of a bulb represents the shortened stem, to which modified leaves are attached and upon which adventitious roots are formed. Typically, an aboveground shoot develops from the apical bud of the basal plate, while daughter bulbs ("cloves", "bulblets") may form in the axils of the scale leaves.

Aboveground modifications include thorns, tendrils, phylloclades, fleshy stems, aboveground bulbils, and tubers.

Thorns are shoots characterized by a sharpened apex, significant lignification, and the absence of leaves. Their position in the axils of leaves indicates their shoot origin. Thorns protect plants from excessive Transpiration and herbivory. They are found in blackthorn, hawthorn, and honey locust.

Tendrils are modified shoots that enable plant stems to attach to a support, occurring in grapes and pumpkins. Strawberry runners are also shoot modifications whose function is vegetative propagation.

Phylloclades are shoot modifications that resemble leaves; however, their positioning in the axils of modified leaves and The Development of flowers on them indicate their shoot nature. Such modifications are characteristic of Ruscus.

Aboveground tubers are modifications typical of kohlrabi cabbage, serving for nutrient storage and forming above the ROOT collar.

Bulbils or mini-bulbs can form either in the inflorescence (bulbous bluegrass) or in the leaf axils of certain plants (bulbous coralroot) and serve for vegetative propagation.

Aboveground modifications also include the stems of succulents, such as American cacti and African spurges, where Water is stored in the stem parenchyma. Succulents are plants adapted to arid environments.