470 THE BEAN PLANT/ [CIIAP. 



contraty direction, from the surface exposed to tlie air, 

 through the air-passages which extend from the stomata 

 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 tem- 

 perature, 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). 



Ik The branches of the stem; some, mere repetitions 

 of the main axis; others, modified and bearing 

 flowers. 



c. The fwdes and internodes. 



d. The appejidages. 

 a. Foliage leaves. 

 /?. Floral leaves. 



b. The root. 



^7. Its main central portion (axis). 



h. The rootlets attached to the axis in four rows. 



c. The root-hairs, only found on the younger parts 

 of the root. 



