188 EXCHANGES AND MOVEMENTS OF FLUIDS 



partial vacuum is indicated. Positive pressures of more than 

 an atmosphere have been measured, and negative pressures in 

 which the gases in the plant exhibited but half of the barometric 

 pressure have been recorded. Interesting results may be attained 

 by the attachment of several manometers to branches of a young 

 tree at various heights, when it may be seen that positive pressure 

 has but little connection with root action. 



261. Guttation, and Action of Nectaries. Grow a number of 

 seedlings of Zea in a pot, and when the blades are a few centi- 

 meters in height, cover with a bell-jar, and note the gathering of 

 drops of water on the tips and margins of the leaves. Note 

 similar appearance of drops of water on margins of leaves of any 

 plant covered with a bell-jar for a few hours. Examine the 

 structure of the nectaries of Passiflora, Cassia, or of any 

 convenient plant and note the structure of the cells lining 

 the nectarial cavity ; under what conditions is nectar ex- 

 creted ? l 



262. Relations of Plants to Gases. Plants are especially con- 

 cerned with oxygen, nitrogen and carbon dioxide, and the diffu- 

 sion of these substances through membranes is governed by the 

 same general laws of osmose, as the passage of liquids. Gases 

 however, penetrate entire membranes only when dissolved in 

 the liquids, with which the membranes are permeated. Carbon 

 dioxide diffuses the most readily, and nitrogen the least readily. 

 Membranes impregnated with wax and other substances, as in cu- 

 ticle and cork, contain but little water of imbibition, and hence the 

 diffusion of gases through such membranes is very slow. The 

 outer covering of the shoots of plants is converted into cuticle or 

 cork in a great majority of instances for protection and conserva- 

 tion of the body of the plant, and this covering is furnished with 

 a large number of openings through the epidermal layers by 

 which the external layer is connected with the spaces between 

 the cells of cortical tissues. Two special forms of such openings 



1 Wieler. Das Bluten der Pflanzen. Cohn's Beit. z. Biol. d. Pflanzen. 6 : i. 

 1*93. 



