30 EXPERIMENTAL PLANT PHYSIOLOGY 



leaf as in the maize (Fig. 13) ; sometimes only on the 

 lower surface as in the privet (Fig. 13) ; but always 

 the amount of transpiration will be proportional to the 

 number of mouths. 



The stomata are bounded by two lips or "cells" 

 called " guard-cells." These guard-cells have the power 

 of opening and closing the pore, and so resemble the 

 lips of our mouths. When they come together the 

 stoma is closed and transpiration ceases; when they 

 separate the stoma is open and transpiration takes 

 place. In this way a plant is able to regulate the 

 amount of water given off. 



Experiment 16 



Aim. — To find out whether the transpired liquid is 

 pure water or whether any of the substances absorbed 

 m solution by the root are lost at the same time. 



Method. — Set up a leafy branch as in Experiment 11. 

 Put some coloured substance {e.g. eosin) into the 

 water in the lower jar. 



Observation. — Tne water that collects on the upper 

 jar is not coloured. 



Inference. — The plant has retained the dissolved 

 substance and the vapour transpired is pure water. 



All the experiments that have now been worked 

 have served to show the close link that exists between 

 the giving off of water-vapour from the leaves and 

 the absorption of solutions by the roots. The conditions 

 that affect one affect the other similarly. If a plant 

 is to grow and thrive, absorption must be somewhat in 

 excess of transpiration. If transpiration exceeds ab- 

 sorption the plant withers. For this reason it is well 

 to protect from the heat of the sun's rays transplanted 

 seedlings whose young absorbing roots must have been 

 injured during the removal. 



The Value of Transpiration to the Plant.— Some 



