Bundle Sheath and Kranz Anatomy
Most minor vascular bundles in angiosperm leaves are surrounded by a bundle sheath which extends even to the very smallest veins (Figs 1.13, 4.2). The developmental origin of bundle sheaths differs; some (mestome sheaths) are derived from procambium or vascular meristematic tissue; others are derived from the ground meristem. The bundle sheath typically consists of thin-walled parenchymatous cells, often in a single layer. Some monocots possess distinct inner and outer bundle sheaths, the outer one being parenchymatous and the (partial) inner sheath sclerenchymatous, forming a sclerenchyma cap that is usually located at the phloem pole. In Aloe the outer bundle sheath is a specialized tissue that is the source of aloin.
Grasses possess either a single sheath consisting of an outer layer of thin-walled cells containing chloroplasts, or a double sheath consisting of an outer layer of thin-walled cells and an inner layer of thicker-walled cells. This is an important taxonomic character in Poaceae, as double sheaths often occur in festucoid grasses and single sheaths in panicoid grasses, though there are exceptions. Leaves of many plants possess regions of
sclerenchyma or parenchyma that extend from the vascular bundle sheaths towards either or both epidermises. These bundle sheath extensions, which are termed girders if they reach the epidermis, afford mechanical support to the leaf and are a xeromorphic feature (Fig. 4.1).
Most plants from warm temperate areas that possess the C4 pathway of photosynthesis display a modified leaf anatomy that is termed Kranz anatomy (Fig. 4.11). Kranz anatomy occurs in a few groups of both dicots and monocots. It is characterized by elongated mesophyll cells that radiate from a single layer of large parenchymatous bundle-sheath cells containing starch and enlarged chloroplasts. This normally forms a second bundle sheath layer, though in some grasses the primary vascular bundle sheath is itself recruited for this purpose. C4 plants concentrate CO2 by photosynthetic carbon assimilation in the radiating mesophyll cells, and subsequent photosynthetic carbon reduction in the bundle sheath cells.