112 THE PLANT 



and he has cited the work of Krober, and of Eberdt in proof. This state- 

 ment holds, however, only for short periods of a few hours, or more rarely, a 

 day, and even here its truth still remains to be conclusively demonstrated. 

 The discrepancy between absorption and transpiration for a short period is 

 often greater than for a longer time, but it is evident that a transient change 

 in behavior or a small error in the method would inevitably produce this 

 result. Eberdt found the discrepancy for a few hours to be 1-2 ccm. in an 

 entire plant of Helianthiis annuns, while for a whole day the water absorbed 

 was 33.57 ccm. and the water lost 33.98 ccm. Krober's experiments with 

 cut branches of Asclepias incarnata showed a maximum difference for 12 

 hours of 2.5 ccm., but the discrepancy for the first 24 hours was i ccm. 

 and for the second 1.9 ccm. In both cases, the potometer was employed. 

 Consequently, as will be shown later, Eberdt's results are not entirely trust- 

 worthy, while those of Krober, made with cut stems, are altogether unre- 

 liable. Kence, it is clear that the discrepancy is slight for a period of several 

 days or weeks, and that it may be ignored without serious error, except in 

 a few plants that retain considerable water as cell sap, in consequence of 

 extremely rapid growth. Accordingly, the amount of transpiration, which 

 may be readily and accurately determined, can be employed as a measure 

 of absorption that is sufficiently accurate for nearly all purposes. The 

 truth of this statement may be easily confirmed. It is evident that the 

 amount of water absorbed equals the amount transpired plus that retained 

 by the plant as cell-sap, or used in the manufacture of organic compounds. 

 In plants not actively growing, the amount lost equals that absorbed, as 

 already shown in the experiment with Helianthus. According to Gain\ 

 Deherain has found that a plant rooted in ordinary soil transpired 680 kg. 

 of water for each kilogram of dry substance elaborated. In Helianthus 

 annnus, the dry matter is 10 per cent of the weight of the green plant. A 

 well-grown plant weighing 1,000 grams, therefore, consists of 100 grams 

 of dry matter and 900 of water. The length of the growing period for 

 such a plant is approximately 100 days, during which it transpires 68 kilo- 

 grams of water. Assuming the rate of transpiration -and of growth to be 

 constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap, 

 and I gram to its dry weight. The amount of water in a gram of cellulose 

 and its isomers is about 3/5. Consequently, the total water absorbed daily 

 by the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent 

 of the amount absorbed; in other words, only 1.4 per cent of the water 

 absorbed is retained by the plant. From this it is evident that the simplest 



^Recherches sur le Role Physiologique de I'Eau dans la V^g^tation. Ann. Nat. 

 Sci., 7:20:65. 1895. 



