THE MOVEMENT OF WATER IN PLANTS. 95 



For Vitis, Hales determined a pressure of 32^ inches of mercury, or 

 825-5 mm. 



It must not be assumed, however, that either the pressure 

 or the rate of flow is uniform. It was observed by Hales that 

 when the column of mercury in the manometer has reached a 

 certain height, it begins to oscillate. We have seen that the 

 height of the column of mercury is a measure of the root- 

 pressure, and therefore its variations must be due to variations 

 of the root-pressure. Hofmeister, in further investigating this 

 matter, made the interesting discovery that these variations 

 have a regular daily periodicity. The column rises in the 

 morning and during the forenoon; then it usually sinks some- 

 what, rises again towards evening, and falls during the night: 

 frequently the slight fall in the afternoon does not take place. 

 According to the more recent researches of Baranetzky and 

 of Detmer, it appears that the maximum is generally attained 

 in the afternoon, though the exact hour varies with different 

 species of plants : in any case there is usually an interval of 

 about twelve hours between the maximum and the minimum 

 flow. The daily period is constant for plants of the same 

 species, provided, however, that they are of the same age, and 

 that the conditions of their growth have been the same for 

 some time before the experiment; in fact, quite young plants 

 do not exhibit a periodicity at all. 



What, now, are the causes of this periodicity? Since we 

 have already learned in a previous lecture that the tempera- 

 ture of the soil has an important influence upon the absorbent 

 activity of the roots, it seems reasonable to suppose that the 

 daily variations of temperature in the soil induce a similar 

 periodicity in the absorbent activity of the roots. This ex- 

 planation is, however, shewn to be incorrect by the fact that 

 the absorbent activity of roots is perceptibly affected only 

 by very considerable variations of temperature of the soil, 

 variations which are much greater than those which take 

 place in nature ; and by direct experiments in which, whilst 

 the temperature of the soil was falling, the rate of flow was 

 found to be increasing. This periodicity is therefore not the 

 immediate result of variations in external conditions ; it is 



