98 LOSS OF WEIGHT. [CH. IV 



mends a Phaseolus grown in a glass vessel having a ground 

 edge so that it can be covered by a divided glass plate. 



We have found it a simple plan to make use of Lambert 

 and Butler's J Ib. tobacco tins. A small plant such as a 

 Pelargonium can be knocked out of its pot and trans- 

 planted to one of these tins. Owing to the stopper-like 

 arrangement by which the tins are closed, it is easy to 

 replace the tin lid by a split cork through which the stem 

 and a watering-tube pass. 



Ascertain the loss by transpiration in say 12 hours, 

 and at the same time ascertain the loss of weight from 

 a shallow dish of water of known area. 



Now calculate the transpiring area of the plant and 

 compare its loss of weight per unit of area with that of 

 the water. 



If a planimeter is not available the area may be 

 calculated by tracing the form of a leaf on stout paper, 

 cutting it out and comparing its weight with that of a 

 piece of known area. If the plant has leaves of various 

 sizes, the leaves are classified into two or three sizes, and 

 the area of one of each heap is taken. If the amount 

 of stem or branches is considerable an estimate should 

 be made of the area of these, and the amount added to 

 the area of the leaves. 



(115) Loss of weight compared with absorption. 



To demonstrate that the loss of weight is roughly 

 equal to the amount of water absorbed by a cut branch, 

 select a branch of Portugal laurel (Prunus lusitanica) 

 which has no young growing shoots, or remove any that 



