. Éléments de biologie ; un texte utile-livre de corréler la botanique, la zoologie et la physiologie humaine. La biologie. 136 Botanique la feuille, on peut enlever les deux feuilles de la même taille et le poids de certaines plantes à grandes feuilles - Une molène a été utilisé pour les illustra- tions étant donné - et couvrir la surface supérieure d'une feuille et la surface inférieure de l'autre avec de la vaseline. Les pétioles de chacun doivent être recouverts de cire ou de la vaseline, et les deux feuilles équilibré exactement sur les plateaux d'une balance qui a déjà été placé dans un endroit chaud et ensoleillé. Dans l'heure la feuille qui a la face supérieure recouverte de va

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. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 136 BOTANY the leaf, we may remove two leaves of the same size and weight from some large-leaved plant — a mullein was used for the illustra- tions given — and cover the upper surface of one leaf and the lower surface of the other with vaseline. The petioles of each should be covered with wax or vaseline, and the two leaves exactly balanced on the pans of a balance which has previously been placed in a warm and sunny place. Within an hour the leaf which has the upper surface covered with vaseline will show a loss of weight. Examination of the surface of a mullein leaf shows us that the lower surface of the leaf is provided with stomata. It is through these organs, then, that water is passed out from the tissues of the leaf. Regulation of Transpiration. — The stomata of leaves close at night. On days when there is little humidity they tend to close, but when the water supply is abun- dant they open. This automatic regulation is of very great impor- tance to the life of the plant, since evaporation of water is thus limited, and consequent wilting of the leaves prevented. The change in the size of the open- ing of the stomata appears to be due to the fact that the protoplasm of the guard cell takes up and loses fluids rather easily. This process we have already noticed under the name of turgor. With an increase in the tur- gidity of the guard cells, which results after an osmotic inflow from surround- ing cells of the leaf, the guard cells change shape so as to increase the size of the opening between them. Simi- fluids from the guard cells, the opening. Diagrams of a stoma; a, surface view of an opened stoma; b, same stoma closed (after Hansen); c, diagram of a transverse section through a stoma — dotted Hnes indicate the closed position of the guard cells, the heavy lines the open condition. (After Schwendener.) larly, with a loss of water or other becomes

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