SPECIFIC POWERS AND ACTIVITIES 147 



Owing to the occasional occurrence of dissociation, a constant relationship 

 does not always exist between the osmotic values of substances and their 

 molecular weights. Thus, using the coefficients given in column IV, the weight 

 of cane-sugar (VI) isotonic with the saltpetre unit may be obtained by multiplying 

 the molecular weight of the cane sugar by f . It is true that the vapour tensions, 

 electric conductivities, &c., of different substances indicate that the relationships 

 existing between the latter cannot always be represented by definite coefficients 

 in the form of whole numbers. Nevertheless, the empirical values (column IV) 

 do not markedly differ from the round numbers found by experiment, at least 

 for the substances given. For physiological purposes approximate values only 

 are necessary, and hence in columns V-VII the values obtained from these 

 coefficients in relation to saltpetre are given. From the considerations already 

 mentioned \ it follows that the usage of these coefficients, and the corresponding 

 osmotic grouping of different substances which they render possible, are of great 

 importance from a physiological point of view. 



From the estimated pressure of cane-sugar (p. 146), and from the relative 

 values given in column VI, the osmotic pressure exerted by solutions containing 

 i grm. in 100 c.cm. of water is calculated in atmospheres and in centimetres of 

 Mercury. The pressures obtained by Pfeffer for gum-arabic and dextrin are also 

 added, and from them the values given in column VI for these substances have 

 been calculated. 



SECTION 25. Specific Powers and Activities. 



In all plants which exhibit functional division of labour, the different 

 members are concerned in varying degrees with absorption and excretion. 

 Such activity is in general determined and regulated by the arrangements 

 and factors already described, i. e. chiefly by the specific needs and meta- 

 bolic activities of the given part, by the readiness with which gases, as well 

 as fluid and dissolved substances, diffuse towards the interior or the exterior 

 of the plant, and by the quality and composition of the surrounding medium. 

 It is obvious that the sub-aerial parts will be chiefly of importance for 

 gaseous exchange, including under this head all volatile materials, while 

 water and dissolved substances must be absorbed mainly by the subterranean 

 or submerged organs. Different members when in contact with water do 

 not necessarily absorb substances in equal amount, for absorption is in- 

 fluenced both by diosmotic properties and by specific metabolic activity. 

 Hence in the cells of a Confervoid filament, provided they are not functionally 



1 The table is compiled from the results obtained by de Vries (Jahrb. f. wiss. Bot., 1884, Bd. xiv, 

 p. 536; Bot. Zeitung, 1888, p. 229). The pressures exerted are given from the values calculated 

 by Pfeffer. 



L 2 



