60 THE CARBOHYDRATE ECONOMY OF CACTI. 



that the cut end of the joint extended into tap-water. It was found neces- 

 sary to allow these cut surfaces to dry out well before placing in water ; in 

 this way any diffusion of sugar from the plant into the water was avoided. 

 Another similar set of joints was placed in the same kind of vessel without 

 water. Within a week the joints in water formed thin roots, which grew 

 15 to 20 cm. in length within the month, but no trace of sugar was found in 

 the water at the end of this time. The experiment was carried out in the 

 dark at constant temperature of 20 C. The analyses are given in table 25. 



From the experimental results just presented it appears that the con- 

 clusions in regard to the effect of water on the carbohydrate-content given 

 in the preceding section are justified. 



The agents effecting these changes of inversion and reversion of the 

 carbohydrates according to the varying conditions of water-supply are not 

 definitely known. The present state of our knowledge of reversible enzyme 

 action offers little enlightenment on the phenomena under discussion. An 

 explanation may be ventured, however, based on the known properties of the 

 plasmic colloids. One of the most striking properties of hydrophile colloids 

 is the avidity with which they take up water and the great pressure de- 

 veloped in the process. Conversely, also, it requires enormous pressures to 

 express imbibed water, the colloids holding the water with tremendous 

 tenacity. 1 Contrary to the water in solutions of crystalloidal salts, the water 

 in these colloidal dispersions is firmly held and has been designated as 

 " hydratation water." Evidence of the avidity with which these colloids take 

 up water can be gained from the fact that they absorb water and swell from 

 concentrated solutions of salts such as calcium nitrate and ammonium 

 sulphate. Now, the inversion of polysaccharides involves the taking up of 

 water into the chemical composition of the resulting simpler carbohydrate. 

 For each molecule of polysaccharide inverted or hydrolyzed, at least one 

 molecule of water is taken up. As the water-content of the plant diminishes, 

 the available water for this process is reduced. If finally the water is so 

 firmly held that it can not enter into the chemical process of inversion, the 

 hydrolysis of the polysaccharides can not take place, or, what is more prob- 

 able, the inversion takes place at a greatly reduced rate. It must soon be 

 realized that existing chemical knowledge is not sufficient to interpret the 

 reactions taking place in living organism. As was emphasized in the intro- 

 ductory discussion, these reactions do not take place under conditions of 

 pure aqueous solution, and deductions based upon such assumptions must 

 prove inadequate in considering the chemical changes in living things. 

 Some knowledge of reactions in colloidal media must supplement the 

 existing physical-chemical conceptions. The formation of pentosans under 

 conditions of decreased water-content, as pointed out previously, is inti- 

 mately associated with the reduced supply of monosaccharides. These 

 phenomena will be discussed under the section on pentose sugars. 



1 RIINKE, J. Untersuchungen ueber die Quellung einiger vegetablischer Substanzen. 

 Haustein's Bot. Abt., 4, Heft 1. 



