TRANSPIRATION. 



279 



In most thin membraneous leaves the upper and under sides are differently 

 constructed, and the difference is not confined only to the cuticle, but is also plainly 

 recognizable in the green tissue. The green cells below the epidermis on the upper 

 side of the leaf have the form of prisms, cylinders, or short tubes, and are arranged 

 very regularly in ranks and files. In the leaves of plants belonging to the lily 

 tribe, they lie with their long axes parallel to the surface; but in most other plants 

 these cylindrical cells have their smaller side directed to the surface, and stand side 

 by side like palisades, with only very narrow air-passages between them. Below 

 these palisade-cells, and bordering on the epidermis of the under side of the leaf, is 

 another stratum of cells of a much looser texture (see fig. 62^). The cells of this 

 under layer are not so crammed with chlorophyll, and therefore appear a lighter 



-y 



£taj W^^, c*)!""^!^^ 



Fig. 62.— Spongy Tissue. 



• Vertical section through leaf of Franeiscea eximia. 2 Spongy tissue in leaf of Daphne Laureola. — The epidermis and 

 palisade cells of the upper side of the leaf are removed. The epidermis of the under side of the leaf, with its stomata, 

 can be seen through the spaces in the spongy tissue; x320. 



green than the palisade-cells. In shape they are elliptical, rounded, angular, sinuous, 

 or generally very irregular; usually they possess protuberances which project in 

 various directions, and they are so arranged that the outgrowths of adjoining cells 

 come into contact with one another. It looks as if the neighbouring cells were 

 stretching out their arms and extending their hands to one another, and consequently 

 these cells have been called " many-armed cells ". When several adjoining stellate 

 cells are connected together in the manner just described so as to form a tissue, 

 lacunae and passages are seen in the tissue, which are broken through by the joined 

 arms of neighbouring cells as if by pillars, couplings, and bridges. The whole tissue 

 has the loose perforated appearance of a bath sponge, and is called accordingly 

 spongy tissue, or spongy parenchyma (see fig. 62 ^). 



This spongy tissue is the proper place for transpiration. Nowhere else in the 

 plant are the conditions governing this process so well fulfilled as just here. The 

 surfaces of the cells are rendered large in proportion to their size by their out- 

 growths; and they impinge as far as possible on the larger or smaller lacunae, 

 gaps, and passages filled with air, which all communicate with one another, thus 

 constituting an unmistakable ventilating system. 



Since the spongy parenchyma in the leaves described does not lie freely exposed, 



