BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Morphology and Anatomy of the Stem and Shoot

Shoot Modifications

Under the Influence of Environmental factors and the performance of various Functions, shoots undergo modification. Both underground and above-ground SHOOT modifications are distinguished.

Underground modifications include the following:

Tubers are modified structures in which the stem thickens due to the accumulation of nutrient reserves. They feature shortened internodes, reduced leaves, a spiral arrangement of "eyes" (buds), deformed Vascular Tissues, and achlorophyllous Cells. Under favorable conditions, these stored nutrient reserves enable them to sprout and give rise to a new plant. Tubers serve as Organs of plant propagation (e.g., potato).

The rhizome is an underground modified shoot, evidenced by its apical growth, division into nodes and internodes, presence of reduced leaves and bract-like arches, remnants of vascular tissues, and The ability to initiate renewal buds. Plants propagate via rhizomes. The ability to form new plants from rhizome fragments 3-4 cm long promotes the mass proliferation of weeds (Fig. 67) (such as couch grass, wood small-reed, ryegrass, horsetail) and forage plant species (such as bromegrass, bentgrass, beardgrass, bluegrass, small-reed, mannagrass).

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Fig. 67. Underground shoot modifications:

1 - bulb, 2 - tuber, 3 - rhizome; a - shoot bud, b - shoot scar, c - stem, d - leaf scars, e - adventitious roots, f - scale leaves, g - axillary buds

The bulb is a modified shoot featuring an extremely shortened stem known as the basal plate (or disk). A bulb possesses fleshy modified leaves (bulb scales) and dry protective outer scales. Axillary buds located in the leaf axils of the fleshy scales give rise to new shoots. The basal plate produces adventitious roots. The bulb functions as both a nutrient storage organ and a reproductive organ (e.g., snowdrop, snowflake, crocus, corydalis, star-of-Bethlehem).

The bulblet is a small, easily detachable above-ground bulb that develops in the leaf axils of toothwort and lesser celandine, serving as a propagation organ for a specific group of plants.

The corm is an underground, shortened, modified shoot of limited growth, covered with scales or leaf remnants. Outwardly, it resembles a true bulb, but it differs by having a thickened, tuber-like stem core. Classic Examples include cyclamen and gladiolus.

Above-ground shoot modifications include the following:

The tendril is a modified shoot that develops from an axillary bud, possesses optimally developed vascular and mechanical tissues, and is capable of apical growth. Examples include the tendrils of grapevine, pumpkin, cucumber, and watermelon.

The spine (thorn) is a modified shoot that serves as an adaptation to reduce Transpiration. It features highly developed mechanical tissue and bears leaves, flowers, and buds, with apical growth—as seen in hawthorn (Fig. 68).

Phylloclades are stem modifications that take on the appearance of leaves. In the Crimean plant Butcher's broom (Ruscus), leaf-like stems emerge. These are green, contain chlorophyll-bearing cells, and perform photosynthetic functions. Phylloclades develop from an axillary bud, as indicated by the scale-like leaf situated beneath the modification. A flower is formed upon it, which is a characteristic feature exclusive to shoots.

Fig. 68. Above-ground shoot modifications:

1 - grapevine tendrils, 2 - lobes of a fleshy stem, 3 - spines, 4 - phylloclade

Modifications in which the stem assumes the function of a leaf include the shoots of cacti, prickly pears, Spanish broom, and others.