This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Leaves may be defined as appendages borne upon an axis but differing from it in structure and organisation. They usually exhibit three regions, viz. a leaf-base, a stalk or petiole, and a lamina or blade. Each of these regions may offer characters that are useful in identifying the leaf. The following schedule may assist the student in examining leaves.
i. Size and Shape. - These may vary within certain, sometimes rather wide, limits. Should the leaf have shrivelled so much as to render them undeterminable it should be soaked in warm water and gently spread out on a white tile.
ii. Venation. - The mode of branching of the midrib, the angle at which the branches (lateral veins) leave it, the course they pursue, and their elevation above or depression below the surface of the leaf should be noted.
iii. Margin. - This margin may be entire, serrate, crenate, dentate, sinuate, etc.
iv. Apex. - This may be rounded, pointed, notched, etc.
v. Base. - The appearance of the base should also be noted.
vi. Surface. - The surface should be examined with a lens, and the presence or absence of hairs, as well as the character of the latter, glandular or otherwise, determined; this particular is of very great importance.
vii. Glands. - Many leaves contain internal oil-glands or oil-cells which can usually be seen as translucent points when the leaf is examined with a lens by transmitted light. In very thick leaves they may be difficult to see, and it may be necessary to examine a section under the microscope.
viii. Texture. - Papery, leathery, fleshy, etc.
ix. Colour. - Greyish green, dark green, etc.
x. Odour. - This is best determined by crushing the leaf between the fingers and smelling it.
xi. Taste. - Often very characteristic.
xii. Petiole. - Presence or absence, length, etc.