78 ELEMENTARY BIOLOGY. [CHAP. 



ment of the oxygen of the air thus absorbed, and the re- 

 moval of the carbonic anhydride formed, will be sufficiently 

 provided for by gaseous diffusion. 



From what has been said, it results that, in an ordinary 

 plant, growing in damp earth and exposed to the sunshine, 

 a current of fluid is setting from the root towards the surface 

 exposed to the air, where its watery part is for the most 

 part evaporated; while gaseous diffusion takes place, in the 

 contrary direction, from the surface exposed to the air, 

 through the air-passages and spiral vessels which extend 

 from the stomates to the radicles ; the balance of exchange 

 being in favour of oxygen, in all the chlorophyll-bearing 

 parts of the plant which are reached by the sunlight, and in 

 favour of carbonic anhydride, in its colourless and hidden 

 regions. At night, the evaporation diminishing with the 

 lowering of the temperature, the ascent of liquid becomes 

 very slow, or stops, and the balance of exchange in the air- 

 passages is entirely in favour of carbonic anhydride; even 

 the chlorophyll-bearing parts oxydizing, while no carbonic 

 anhydride is decomposed. 



LABORATORY WORK. 

 a. General characters. 



a. The erect central main axis (root and stem}. 



b. The branches: some, mere repetitions of the 

 main axis ; others, modified and bearing flowers. 



c. The nodes and internodes. 



d. The appendages. 

 a. Rootlets. 



/?. Foliage leaves, 

 y. Floral leaves. 



