The epidermis and its modifications, such as the hypodermis and periderm, form the protective outer layer or layers of plants. The epidermis of most leaves, stems of herbs, seeds, fruits, floral organs, and young woody stems consists of a single layer of cells which form an impervious covering with stoma exceptions.
LEAF EPIDERMIS: Cells vary in size, thickness, form, arrangement, outgrowths, surface deposits, wall character (smooth or rough), and color. In cross-sections, side walls are more conspicuous than end walls due to light retardation effects. Outer walls of epidermal cells are characteristic when striated, rough, pitted, colored, etc., but not diagnostic in powders.
The thickness of the end and side walls differs greatly among plants; aquatic and shade-loving plants have thinner-walled epidermal cells than those growing under normal conditions or shrubs/trees. The widest range of cell-wall thickness is found in medicinal leaves due to diverse origins.
In most leaves, there are five typical forms of arrangement: over veins (elongated), other parts (several-sided), around stomata, surrounding hair bases, and outgrowths like non-glandular and glandular hairs. The character of the outer wall varies greatly among plants; smooth walls in senna, rough striations in belladonna.
Surface deposits are uncommon but include waxy deposits on sumac stems and raspberry fruits, resinous deposits on grindelia leaves and yerba santa stems. Hypodermal cells occur beneath the epidermis, especially at leaf margins (e.g., uva-ursi).
TESTA EPIDERMIS: Epidermal layers of seeds like lobelia, henbane, capsicum, larkspur, belladonna, scopola have thick and wavy end walls. Walls are porous or non-porous with varying colors from yellow to brown.
PLANT HAIRS (trichomes): Plant hairs offer a ready means for distinguishing plants in broken or powdered condition. They vary greatly by species, size, form, arrangement, color, wall character, branching, curvature, and tip shape.
Papillae occur on flower petals, stigmas, styles, leaves, stems, seeds, fruits; they are small tubular outgrowths of epidermal cells with thick outer walls. Papillae vary in size and form among different flowers.
UNICELLULAR NON-GLANDULAR HAIRS: Common to many plants, these hairs occur solitary or clustered on leaves (chestnut, yerba santa), fruits (anise), stems (allspice, senna). They are smooth or rough with thick walls, often containing reddish-brown tannin.
MULTICELLULAR HAIRS: These include uniseriate and multiseriate hairs. Uniseriate non-branched hairs occur on digitalis, skullcap, peppermint, thyme, yarrow, arnica flowers, sumac fruit; they are rough or smooth with varying numbers of cells.
<Callout type="important" title="Key Identification Feature">Plant hairs offer a critical diagnostic tool for identifying medicinal plants in powdered form.</Callout>
Multicellular multiseriate non-branched hairs occur on cumin fruits and calendula corolla; they are spreading at the base, blunt or rounded at the apex. Multicellular uniseriate branched hairs occur on dittany of Crete leaves, mullein, lavender calyx.
Key Takeaways
- The epidermis is a protective layer in plants that varies significantly among different species.
- Plant hairs are crucial for identifying medicinal plants, especially when they are broken or powdered.
- Hypodermal cells and papillae provide additional diagnostic features for plant identification.
Practical Tips
- Use the presence of specific types of trichomes to identify medicinal herbs in their natural state.
- Examine leaf margins closely as hypodermal cells often occur there, aiding in species differentiation.
Warnings & Risks
- Do not confuse different plant parts with similar epidermal structures; this can lead to misidentification and potential misuse of plants for medicinal purposes.
- Be cautious when handling plants with rough or sharp trichomes as they may cause skin irritation or injury.
Modern Application
While the historical focus on detailed botanical anatomy is foundational, modern survivalists benefit from understanding these structures for plant identification in the wild. Knowledge of epidermal characteristics and trichome types remains crucial for distinguishing between medicinal plants and potentially harmful look-alikes.
Frequently Asked Questions
Q: What are some key features to look for when identifying medicinal leaves?
Key features include the thickness and character of cell walls, presence of stomata, arrangement of epidermal cells, and specific types of trichomes.
Q: How do plant hairs assist in botanical identification?
Plant hairs are highly diagnostic as they vary greatly by species. Their size, form, arrangement, color, wall character, branching, curvature, and tip shape can help identify different plants even when powdered or broken.
Q: What is the significance of hypodermal cells in plant identification?
Hypodermal cells often occur at leaf margins and provide additional diagnostic features for distinguishing between species. Their presence and characteristics are important indicators of specific plant types.