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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 smaller. 



The Effect of Transpiration on Water within the Stem. — It has 

 already been noted that root pressure alone will not account for 

 the rise of water to the tops of very tall trees. The following 

 experiment shows that transpiration exerts a lifting power upon 

 the fluids within the stem: — - 



