The leaf anatomy of hemlock, Tsuga canadensis

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Abstract
Structural features related to the pathway of water movement in the leaf of hemlock, Tsuga canadensis (L.) Carr., have been examined in conjunction with investigations of plant water relations. Cell surface areas and volumes were determined by projecting 2-μm serial sections, measuring them with a map tracer, and multiplying cell perimeters by section thickness; or by the trace–cut–weigh method. Scanning electron microscope observations showed that the stomata are occluded with a continuous layer of rod-like wax particles. The average maximum distance over which water must move from the xylem of the single median bundle to the mesophyll is 770 μm, and each 40 μm of length of vein (the diameter of a mesophyll cell) supplies 196 cells. The palisade mesophyll consists of two layers, while the spongy mesophyll is arranged in sheets which radiate from the vascular bundle to the margin and alternate with large intercellular spaces. The mesophyll cell walls may provide the pathway through which the bulk of the water moves; the volume occupied by the mesophyll cell walls is 1.27 × 10−4 cm3 or 2.5% of total leaf volume. The total evaporative surface area of the mesophyll is 1.61 cm2 per leaf.

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