PLANT PHYSIOLOGY 133 
tight the water will all stay in the tube. Fix it to the 
stand, as in the diagram, with the lower end dipping 
into a cup of mercury. 
Result.—In a few minutes the mercury begins to rise 
up the tube ; it may rise as much as 5 inches in an hour; 
this will depend partly on what shoot is used, and 
partly on the temperature. 
Since mercury is thirteen and a half times as heavy 
as water, this experiment shows that a leafy shoot is 
able to lift up a considerable weight of water. The 
transpiration current may also be shown to exist by 
cutting some leafy shoots and letting them dip into 
red ink. Ina short time the veins will all be coloured 
red. 
101. Transpiration Current.— This TRANSPIRATION 
CURRENT is very important in causing the distribution 
of mineral substances throughout the plant. The rate 
of transpiration is regulated by the stomata. Thev close 
in darkness, for then less food is needed, and by so 
doing they also prevent the plant getting too cold, as ex- 
cessive evaporation lowers the temperature considerably. 
They also close if transpiration is too active, i.e. if 
more wateris being lost than the roots can absorb. On 
the whole transpiration is more rapid in sunlight, and 
in a strong wind, than in shade or still air. This can 
easily be verified by a modification of Experiment 9. 
If four bottles are fitted with leaves as described, and 
weighed, and then one is placed in sunlight, one in the 
dark, one in the shade, and one in a direct draught, 
and then weighed again, it will be found that the per- 
centage loss is greatest in Nos. 1 and 4, and least in Nos. 
