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which it can support, if the apparatus is left for some time it will 

 be found that the level of the mercury will not be constant, but 

 will oscillate slightly above and below the level at first attained ? 

 but will not deviate much from it on either side. These varia- 

 tions, it will be found, take place in a definite sequence, the en- 

 tire round of which is accomplished every twenty-four hours, 

 and to these variations in the root pressure the term Daily 

 Periodicity has been applied. At first sight, this daily periodi- 

 city would seem to be due to external changes, i.e., changes 

 in the temperature of the soil and the air, produced by the 

 alternations of day and night, affecting the root. There are 

 reasons for believing that it is not due to this source, how- 

 ever, since we find a slow rise of the root pressure at the 

 same time that we get a decline of temperature ; and, further, 

 the temperature of the soil never varies to such a degree in 

 twenty-four hours as is required to produce these oscillations. 

 Hence we may conclude that the temperature of the air and the 

 soil has no important bearing on this daily periodicity. The 

 influence of light appears the next likely agency to produce it ; 

 but it happens that if we put the plants in the dark these oscil- 

 lations will go on just as well as they do in the light. It is not 

 due, then, to variations in external conditions, but to variations 

 in the absorbent condition of the root-cells. It is probable that 

 these latter variations were induced in bygone times by varia- 

 tions in external conditions, and that these have become 

 transmitted by heredity, so that the roots of the plant have 

 become habituated to vary in their absorbent activity, and 

 hence these variations can at the present time go on quite 

 independently of the action of external conditions, e.g., in the 

 dark. 



In herbaceous plants, the water forced up from the roots 

 contains only mineral salts in solution taken up from the 

 soil. In the Vine, Maple, and some trees, it usually holds 

 in solution various organic substances, and particularly sugar, 

 which it obtains from the stem. 



In winter, the wood-cells and vessels contain water, together 

 with larger or smaller bubbles of air ; hence, if a hole be 



