rUNCTION.S OF LEAVES 



133 



surfaces. Stomata occur on the epidermis of yoimg stems, 

 being replaced later by the lenticels. Those plants which, 

 like the cactuses, have no ordinary leaves, transpire through 

 the stomata scattered over their general surfaces. 



EXPERIMENT XV 



Amount of Water lost by Transpiration. — Procure a thrifty hydran- 

 gea^ growing in a small flower-pot. Calculate the area of the leaf- 

 surface by dividing the surface of a piece of tracing cloth into a series 

 of squares one-half inch on a side, 

 holding an average leaf against this, 

 and counting the number of squares 

 and parts of squares covered by the 

 leaf. This area multiplied by the 

 number of leaves will give ajpproxi- 

 mately the total evaporating surface. 



Transfer the plant to a glass bat> 

 tery jar of suitable size. Cover the 

 jar with a piece of sheet lead, slit to 

 admit the stem of the plant, invert 

 the jar, and seal the lead to the 

 glass with a hot mixture of beeswax 

 and resin. Seal up the slit and the 

 opening about the stem with graft- 

 ing wax. A thistle-tube, such as is 

 used by chemists, is also to be in- 

 serted, as shown in Fig. 95.^ The 

 mouth of this should be kept corked 

 when the tube is not in use for 

 vratering. 



A Hydrangea 



for Exp. XV. 



iThe common species of the green- 

 houses, Hydrangea Hortensia. 



2 It will be much more convenient to tie the hydrangea if one has heen 

 chosen that has but a single main stem. Instead of the hydrangea the com- 

 mon cineraria, Senecio cruentus, does very well. 



