52, 



Methods of Experimentation 



Three experiments were performed on detached leaves 

 of P. polypodioides » In the first experiment four groups 

 of four leaves each were placed in a desiccator containing 

 a uniform layer of calcium chloride, In the first group 

 each leaf received a thin layer of petrolatum on hoth the 

 upper and lower epidermis thus sealing up both surfaces 

 completely. The leaves of the second group received a layer 

 of the petrolatum on the upper surface only , leaving the 

 lower one open; those of the third group were sealed only 

 on the 3i07/er side, leaving the upper side untouched; tut 

 the leaves of the fourth group were left in the natural 

 condition so that transpiration could take place normally 

 from "both the upper and lower surfaces .In each case only 

 the leaf blade was used, in order to avoid einy complications 

 arising from an uneven sealing of the cylindrical petiole. 

 After the sealing of each leaf, it was 7/eighed and then dried 

 in the desiccator for twenty four hours and weighed again 

 at the end of that time, The weighing was repeated dally for 

 one week.At the end of the experiment the recorded weights 

 of each group were averaged and the percentage of water lost 

 from each surface was calculated from the results obtained. 

 Having thus determined the rate of water loss from each 

 surface of the leaves, these same leaves were then suspended 

 from wires in ahoist chamber so that ea.ch leai" was a centi- 

 meter or so above the v/ater level but not actually in contact 



