. Principles of the anatomy and physiology of the vegetable cell. Plant cells and tissues. ^9 ANATOMY AND PHYSIOLOGY OF Mg. 35. Intercellular passages occur mostly between parencliymatous cells; they are frequently absent from prosenchyma, or when pre- sent, are, at least, very narrow. They are closed in most places at the surface of the plant, since the parenchymatous cells which form the outermost layer of the plant are, in general, and in all parts growing under ground or in water without exception, accurately in contact at their angles ; on the other hand, on most organs ex- posed to the a
. Principles of the anatomy and physiology of the vegetable cell. Plant cells and tissues. ^9 ANATOMY AND PHYSIOLOGY OF Mg. 35. Intercellular passages occur mostly between parencliymatous cells; they are frequently absent from prosenchyma, or when pre- sent, are, at least, very narrow. They are closed in most places at the surface of the plant, since the parenchymatous cells which form the outermost layer of the plant are, in general, and in all parts growing under ground or in water without exception, accurately in contact at their angles ; on the other hand, on most organs ex- posed to the air, especially on the lower sides of leaves, there occur little ox^ifices bounded by crescent-shaped, curved cells, sto- mates or stomata (fig. 85), which allow a free communication between the air contained in the intercellular passages and the atmosphere. The more regularly polyhedral the cells are, the more do the intercellular passages take the form of regular, nar- row canals (see fig. 7); on the other hand, the more globular the shape of the cells (fig. 6), and in a still higher degree, the more an unequal growth has caused them to approach the form of the stellate cell (fig. 10), the more do the intercellular passages take the form of irregular cavities, and the more spongy becomes the tissue of the organ formed of such cells, since the space occupied by the intercellular passages then becomes more equal to, or in ex- treme cases, many times surpasses, that filled by the cells. The lower side of leaves and corollas are formed of such tissue, moderately spongy, the pith of Juncus effusus gives a very highly developed example. In other cases the intercellular passages lying between regular polyhedral cells become expanded at particular points into larger cavities, or into long canals, which latter are frequently interrupted at certain distances by partitions composed of stellate cells. This is the case in the stem and in the leaf-stalk of many water- and marsh-plants, in w
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