76 ELEMENTAEY BIOLOGY. [VIII. 



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. 



/3. Foliage leaves. 

 7. Floral leaves. 



b. The root. 



a. Its main central portion (axis). 



b. The irregularly arranged rootlets attached to the 

 axis. 



c. The absence of chlorophyll in the root. 



