Sampson tvid Allen: physical factors on transpiration. 35 



being so enclosed as to allow no loss of water except through trans- 

 piration. Seeds were sown in flats and the young seedlings trans- 

 planted into pots. For individual measurements only one plant was 

 grown in a pot, but in working with soil types and textures where 

 wheat alone was used, four plants were grown in the pot. 



For the soil tests, earthern pots dipped in hot parafifin were used 

 exclusively. The water content of the soil in the pots was kept as 

 constant as possible. No additional water was added to the soil 

 during the experiment. In the case of either wire or earthern pots, 

 the tops were covered by parchment paper which had been dipped 

 in hot paraffin. The paper was cut to fit the pot and tightly sealed 

 by running paraffin around the edges. 



Only the leaves were considered as transpiring surfaces, since 

 it was found that the stems lost only two to three per cent of tht 

 total amount. Leaf areas were determined first by simply tracing 

 the leaves on paper and cutting out the traced portion. Several 

 units of paper were weighed, an average taken, and the area calcu- 

 lated. Later photographic prints were made of the leaves, vising 

 solio printing paper. The white portion representing the leaves 

 was cut out and weighed. Knowing the weight of one square deci- 

 meter of solio paper, the area was readily calculated. The area 

 thus obtained is, of course, that of one side of the leaf only, and 

 must be doubled for the total area. 



Several weighings were made to determine the uniformity in 

 weight of the paper, and an average of the readings was used 

 throughout. The readings and average thereof are shown in the 

 following table : 



Solio per sq. dec No. I 1685 mg- 



Solio per sq. dec No. II 1692 mg. 



Solio per sq. dec No. Ill 1690 mg. 



Solio per sq. dec No. IV 1693 mg. 



Solio per sq. dec No. V 1694 mg. 



Average 1690.8 mg. 



From the above average, 16.9 mg. was taken as the weight of i 

 sq. cm. of paper. The weight of the leaf prints divided by 16.9, 

 i. e., the weight of i sq. cm. multiplied by 2, gives the leaf area in 

 square centimeters. 



Stomata. In all plants used, stomata counts were made ^ by 

 peeling off a small part of the epidermis. An ocular micrometer 

 marked off in quadrants was used, making it possible to get an ac- 

 curate count. Three counts were made on each leaf, one reading 



1 Exception — HoUdiscus. 



