Berthelot's 

 principle dis- 

 tinguishes 

 the water of 

 hygro- 

 scopicity 

 from the 

 water of 

 vitality. 



The simplest 

 mode of 

 stating the 

 water-con- 

 tents of 

 plant-tissues. 



152 STUDIES IN SEEDS AND FRUITS 



putting this question to myself as to the significance of the 

 gain in weight of plant-tissues, and more particularly of seeds 

 after being exposed to desiccation in the oven, never dreaming 

 that such a simple experiment would supply the answer. To 

 have been contented with attributing it to hygroscopicity would 

 have explained little. As a disconnected fact it appeared 

 without interest and without meaning. What was required 

 was the establishment of a relation between this property and 

 some other attribute of plant-tissues ; and this I ultimately 

 found in the principle of Berthelot. 



From this standpoint the water-contents of plants could be 

 divided into two parts, the water of hygroscopicity and the 

 water of vitality. The first, being independent of life, is 

 equally characteristic of the plant living and the plant dead. 

 It is the rpsiduum left in the air-dried material ; and it is the 

 water that the same material loses in the oven and regains 

 in the air. The second is the water that distinguishes the 

 plant as a living organism. Its quantity is regulated only 

 by the needs of that organism. Unlike the water of hygro- 

 scopicity, it does not directly respond in its variations to 

 the hygrometricity of the air. On the other hand, hygro- 

 scopicity being a non-vital process represents the response of 

 the non-vital part of a plant's water-contents to the varying 

 humidity of the atmosphere. We can thus understand 

 how the residuum of water in the air-dried plant is the 

 water that represents a function of the hygrometric state of 

 the air. 



This mode of differentiating the water-contents of plant- 

 tissues is of practical importance. There are many ways of 

 stating the proportions, and I have spent much time in trying 

 to harmonise them with the results of my observations. 

 Finally, it became evident that of the numerous methods of 

 describing the " hydratation " of plants there were none so 

 simple and none so true as that implied in the principle of 

 Berthelot. There is the water that the organised tissues 

 contain, whether living or dead, the water of hygroscopicity ; 



