. Anatomie végétale du point de vue du développement et les fonctions des tissus, et le manuel de micro-Technic. bres dans theleaves, où ils font la moyenne de 100 à 300 milli-carré mètre ; et parfois, comme inspecies d'Olea et Brassica,ils deviennent aussi nombreux que600 à 700 par m2 d'eau Milli-mètre. Une stomie est gardée par les cellules twoguard (Fig. 62), ce qui, en règle générale, ont le pouvoir d'ouvrir et de fermer automatiquement. thestoma La formation d'un stomate est fourni par la division d'un protodermal cellinto mère deux cellules filles et thedissolution middlelamella du mur de l'separatingthese, ce f

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. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. bers in theleaves, where they average100 to 300 per square milli-meter; and sometimes, as inspecies of Olea and Brassica, they become as numerous as600 to 700 per square milli-meter. A stoma is guarded by twoguard cells (Fig. 62), which, as a rule, have the power automatically to open and close thestoma. The formation of a stoma comes about by the division of a protodermal mother cellinto two daughter cells and thedissolution of the middlelamella of the wall separatingthese, thus forming a crevice, which may be closed, or openedout by the action of the guardcells to a breadth of approx-imately .008 mm., or aboutone-nineteenth the thickness ofthis page (Fig. 63). So minuteare these clefts in the epi-dermis, even in their wide-open condition, that their valueas entrance ways for carbondioxide gas cannot be appre-ciated without experimentaldata. This, fortunately, is athand. It has been found in purely physical experiments thatthis gas will diffuse at a faster rate through very minute open-. FIG. 63.—A, diagram showing relativeposition of the guard cells in cross section inthe open and closed positions; the heavierline indicates the open position. B and C, early stages in the formation of stomata;at 5, mother cells of guard cells are shown.D, s, s, two guard cells formed by thedivision of a mother cell. (After Sachs.) 126 INTAKE AND DISTRIBUTION OF GASES ings in a membrane than through larger openings; and byhaving the minute openings frequent enough it is possible for itto pass through them as rapidly as if no membrane whateverwere interposed. Physiological experiments show further thatwhen the stomata are closed carbon dioxide does not enter theleaf rapidly enough for food-making. (See Chapter IX.) A better conception of the frequency of the stomata will begained by a comparison of a and b, Fig. 64. a, which is a square of 5 mm. on the side, contains 100

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