Chap, i Processes of Absorption of Water 257 



showed that a similar pumping action of turgid cells takes 

 place in severed portions of plants, such as grass-haulms, 

 when they are supplied with water, and that the exudation 

 of drops of water by herbaceous plants like Alchemilla has 

 a similar cause. Sachs attributed also the excretion of 

 nectar by flowers to hydrostatic pressure following osmotic 

 absorption, a view which was disproved by Pfeffer in his 

 Osmotische Untersuchungen in 1877, and replaced by the 

 theory of secretory activity. 



Pfeffer found that the feebly-active nectaries of the 

 Cherry-laurel will continue to excrete a syrupy liquid if 

 they are kept in moist air, though the leafy branches may 

 have lost more than a quarter of their water. The process 

 is consequently to be recognized as a vital one, depending 

 on the activity of the gland cells. This is supported by 

 the observation of Darwin in 1877, that light plays a great 

 part in starting and maintaining the secretory flow. Wilson 

 showed that temperature also is concerned in the mechanics 

 of the exudation. 



The amount of root-pressure exhibited by a plant has 

 been shown to undergo daily and annual variations which 

 are irregularly rhythmic. Hofmeister first established the 

 daily periodicity in 1862, but it was more carefully studied 

 by subsequent observers, of whom the most prominent was 

 Baranetzky, whose treatise Eine Mittheilung fiber die Perio- 

 dicitdt des Blutens appeared in 1873. He determined that in 

 any given case a maximum flow can be noticed at some defi- 

 nite time of day, which varies with different plants, and he 

 attributed it ultimately to the alternation of day and night, 

 for he found that any alteration in the periods of illumina- 

 tion causes the daily periodicity to change. Baranetzky 

 observed further that this periodicity is not without excep- 

 tion, not being shown at all by some plants. Brosig 

 followed up the subj ect in 1876, and Detmer in 1877. A more 

 important contribution to our knowledge was made by 



GREEN R 



