92 VEGETABLE PHYSIOLOGY 



being taken every hour ; or in the first form of the appa- 

 ratus the manometer can be fitted with a float carrying 

 a pen, which can be made to trace a continuous line on a 

 slowly rotating recording surface. The line will be found 

 to describe a curve, showing points of activity varying from 

 maximum to minimum. The general features of the curve 

 will be the same for all plants, but all do not give the 

 maximum at the same time of the day. In the case of 

 Cucurbita Melopepo the minimum point occurs in the early 

 morning ; the curve rises slowly during the forenoon, 

 reaching its maximum soon after midday. From this 

 point it falls ; sometimes a second smaller rise takes place 

 towards evening, and then it sinks continuously all night. 

 The time of the occurrence of the maximum point varies 

 in different plants, but in all it appears to be during the 

 afternoon. In Prunus Laurocerasus it is much later than 

 in Cucurbita. The points of maximum and minimum 

 activity appear, however, to be about tw r elve hours apart, 

 so that there is a complete diurnal cycle. 



There may be noticed in some trees also a variation 

 which suggests a yearly periodicity. The power of exud- 

 ing water is lost for a time during the winter, the loss 

 being noticeable at different times in different trees. Vitis 

 vinifera does not show any exudation usually in January ; 

 Acer platanoides is passive in November ; many plants will 

 not bleed at all during the winter. 



The causes of these variations in the activity of the 

 absorbing mechanisms of the roots are still obscure. The 

 annual periodicity, when it exists, appears to be connected 

 with conditions which lead to the discontinuance of growth 

 during winter. The trees pass in fact into a state that may 

 be compared to hibernation. The daily periodicity does not 

 appear to depend upon variations in the surroundings of 

 the plant, but to be due to some cause or causes inherent 

 in its constitution. It has been suggested that it has been 

 induced in plants by long-continued variations of external 

 conditions, particularly those of illumination, involved as 



