110 Plant Physiology 



to the leaf with plasticene or prepared wax. In this experiment 

 handle the paper with forceps, and preferably use a leaf attached 

 to the plant, or a shoot, the stem of which is immersed in water. 

 Note any change of color, and the time required to produce 

 change, in the two pieces. Experiment with several of the 

 plants mentioned in section 53, and contrast your data with the 

 indications regarding stomata there furnished. 



Amount of transpiration, determined by weight. - The actual 

 transpiration of potted plants may be carefully determined by 

 loss of weight, as already indicated. Employ plants of any 

 kind convenient, preferably one-stemmed plants with large, 

 relatively simple leaves ; inclose the pot in soft rubber cloth, in 

 aluminium shells and rubber cloth, or in any manner convenient 

 to prevent evaporation from the pot and soil, the plant being 

 previously watered. Weigh carefully and repeat the weighing 

 after each of several intervals of not more than twelve hours. 

 If water is again applied, add approximately the quantity lost, 

 and weigh again. Plot the results. This experiment may be 

 extended through a considerable period of time, and different 

 types of plants may be contrasted. Ultimately, the area of 

 each plant must be taken into consideration or unit areas com- 

 pared, as indicated in later experiments. 



The transpiration of plants grown in water cultures in par- 

 affined wire baskets, or in glazed pots covered with paraffin, 

 is also conveniently determined by weighing, as referred to in 

 subsequent sections of this book. 



Measurement of leaf areas. - - It would be difficult to deter- 

 mine directly the transpiration of a tree or of any vegetation 

 under natural conditions. For the laboratory experimental 

 work in contrasting different plants or plants under different 

 conditions, as well as for an indication helpful in estimating 

 water-loss in the field, it is desirable to have a quick method of 

 measuring leaf areas. 



Many methods of determining leaf areas are now used. Ordi- 

 narily it is sufficient to trace the outline of the leaf upon coordi- 

 nate paper, the area being determined by a count of spaces. 

 Another simple method is to trace the outlines of the leaves 



