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

VII. PLANT TISSUES

Excretory (Secretory) Tissues

Excretory Tissues of External Secretion

They are represented by unicellular or multicellular structures, which include glandular hairs, glandular scales, glands, osmophores, nectaries, and hydathodes (Fig. 52). Originating from epidermal or subepidermal Cells, they secrete Essential Oils, nectar, Water, and mucilaginous substances.

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Fig. 52. Excretory Tissues of external secretion:

1 - glandular Hair of a pelargonium leaf petiole, 2 - capitate hairs, 3 - stinging hair of a nettle, 4 - glandular scale of a chamomile leaf, 5 - glandular scale of a peppermint leaf, 6 - secretory tissue of flower osmophores, 7 - hydathode in a Chinese primrose leaf.

Glandular hairs consist of a single- or multicellular stalk and a single- or multicellular HEAD. For instance, in peppermint, the glandular hair features a unicellular stalk with a spherical head, whereas in sage, it has a one- to three-celled stalk with a spherical unicellular head. Purple foxglove possesses Two Types of glandular hairs: one with a unicellular stalk and a bicellular head (figure-eight-shaped), and another with a three- to five-celled stalk and a unicellular head. Belladonna also has two types of glandular hairs—one with a multicellular stalk and a unicellular head, and another with a unicellular stalk and a multicellular head. On henbane leaves, glandular hairs feature a multicellular stalk and a multicellular oval head. Datura glandular hairs have a short, curved unicellular stalk and a multicellular obovoid head.

Glands. There is no sharp boundary between glandular hairs and glands. Thus, in Lamiaceae and Asteraceae, glandular scales are referred to as glands. They have a short stalk and an inflated head. In Lamiaceae, the glands are located in depressions on the leaf surface and consist of a short unicellular stalk and eight (more rarely, four or twelve) excretory cells radiating from it. In Asteraceae, the glands are composed of two rows of cells arranged in three to four tiers. Glandular hairs and glands secrete essential oils, resinous substances, Gums, mucilages, and the like.

Hydathodes are water Stomata that excrete water from the plant in liquid form. This process is known as guttation. Hydathodes are located along the leaf margins. They consist of guard cells bordered on the inside by loosely arranged mesophyll cells that form the epithem. The latter is in close contact with the xylem part of the vascular bundle.

Nectaries are multicellular glands that produce and secrete nectar. They can be located within flowers (floral nectaries) or on vegetative Organs (extrafloral nectaries). Floral nectaries occur on petals, at the base of stamens, on anthers or staminodes, around the gynobase disc, or on the pistil Ovary. Extrafloral nectaries are found on stems, leaves, stipules, and pedicels. Nectary tissue is formed by epidermal or both epidermal and subepidermal cells adjacent to conducting tissues. Nectar may be secreted outward through the stomatal pore between the epidermal guard cells or directly through The Cell walls of the epidermis.

Osmophores are specialized flower glands that secrete volatile Aromatic Compounds. Their tissue consists of a secretory epidermis and several adjacent cell layers. Osmophores function exclusively during blooming, attracting pollinators with their scent.

Halophytes (plants of saline habitats) can excrete salts through their glandular structures.