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 
& 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 twelve 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 paxs 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 
