180 THE PLANT CELL. 



internal tissue, and is thus directly available. The system of 

 spaces lying amongst the spongy-parenchyma of a bifacial leaf is 

 an instance of a similar provision for adequate aeration. 



c. The assimilation of COj or rather the intake of COj . HgO 

 ( = carbonic acid) into plant-cells for the purposes of sugar- 

 formation is demonstrated by placing a quickly-growing plant in 

 a vessel, which is then filled with a mixture of air moisture 

 and CO2 in known proportions, the COj being in excess of what 

 it is in atmospheric air. This vessel is then placed in sunlight 

 for about six hours, and at the end of that time the experiment 

 stopped and the gases in the vessel tested. 



It will be found that as a result of the intake of COg into the 

 mesophyll cells of the leaf by way of the stomata, there is less 

 CO2 in the vessel in proportion, and more oxygen. The respira- 

 tory constants of the plant in daylight should be known, but the 

 error caused by the gases interchanged during respiration is a 

 small one. 



These three experiments show that a continuous gaseous 

 interchange is taking place between the leaf-cells of a living 

 plant and the surrounding atmosphere (or water), and that 

 the cell requires oxygen for purposes of respiration or tissue- 

 oxidation. Plants surrounded by an inert gas, such as nitrogen 

 or hydrogen, die in a short time; and moisture must also be 

 present in the air, for the COg and Oj gain entrance into a cell 

 dissolved in water, the former as a definite compound, COg . H.^O, 

 or carbonic acid. 



vii. Variations of Protoplasmic Activity under Different 



Conditions. 

 Under this heading will be considered : — 



a. The influence of light upon the direction of growth of 



organs. 



6. The influence upon metabolism of light rays of varying 

 refrangibility. 



c. The action of gravity or a centripetal force in any direction 

 upon the direction of growth. 



d. The influence of mechanical and Chemical stimuli upon 



protoplasm. 



a. The influence of light-rays (direct sunlight especially) upon 

 growing organs is usually referred to under the comprehensive 



