Elementary biology : descriptive and experimental / by John Thornton.

  • Thornton, John
Date:
1907
Catalogue details

Licence: In copyright

Credit: Elementary biology : descriptive and experimental / by John Thornton. Source: Wellcome Collection.

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    vary greatly in form, and are capable of many modifications and adaptations. The edges of the blade may be entire or indented in various ways, while the surface may be smooth or covered with hairs or down. The youngest leaves are always nearest the apex of a stem or branch, and the oldest nearest the base. New leaves never originate below those already formed. A fully developed leaf consists of two chief parts—a leaf- stalk of hard substance called the petiole, and an expanded portion on each side of a midrib called the blade or lamina. The midrib is con- tinuous with the petiole or leaf- stalk, and gives off branches on each side called veins, these veins subdividing again and again. Between the veins of hard substance lies the softer tissue making up the bulk of the leaf-blade. At the base of the leaf-stalk there are often found two leaf-like appendages on each side called stipules. The mode in which the veins of a leaf are arranged is termed the venation. In Dicotyledons (par. 29) the veins are much branched and form a regular network, so that such leaves are said to be net-veined or reticulate (Lat. rete, “ a net ”). In most Monocotyledons the veins arc more or less parallel, and they are said to be parallel-veined. When Fig. 15.—Skeletons of leaves to show (a) branching net veins, (b) veins parallel to centre.
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