I HE PERIODICITY OF BLEEDING 265 



water for a certain period during winter. Under normal conditions Vitis 

 vinifera was usually unable to bleed in January, and Acer platanoides in 

 November, whereas plants of Betula alba for the most part did not bleed 

 in November and December, Ampclopsis quinqtiefolia not from November 

 to April, Popnlus canadensis not from August to May. The maximal 

 power of bleeding occurs in spring, and presumably the exudation of water 

 is most active when the pressure causing it is greatest, although it is by no 

 means easy to construct a curve from the relative amounts of sap which 

 escape from a plant at different times of the year. 



This periodicity is without doubt simply a direct expression of the 

 internal changes which are repeated every year in hibernating trees, 

 and since we are dealing with a single function only, it does not follow that 

 a stoppage of growth must necessarily cause a disappearance of the power 

 of exuding water. The plant continues to respire throughout the entire 

 year, though not always at the same rate, and in the same way it can 

 respond to an injury, although the amount of response possible is not always 

 the same. It is possible that plants may exist which are always capable 

 of bleeding, and indeed some of the above-mentioned plants can exude 

 water when their winter sleep is deepest. According to Wieler, the highest 

 exudation-pressure does not always exist at the time when the buds are 

 expanding and new roots are being formed *. The fact that the highest 

 pressure is exhibited in spring (Sect. 42) shows that the phenomenon 

 is of considerable importance in the plant's economy, especially at this 

 period of the year. 



When a plant is transferred to a country where the seasons are different, 

 or to the tropics, where the seasonal differences may be but slight, the 

 periodicity under discussion will probably ultimately become modified, and 

 in the plants of a moist tropical climate we may safely conclude that the 

 power of exuding water will either be entirely absent or present all the year 

 round. When an exudation of water is excited in winter, it does not 

 necessarily follow that the winter rest will be broken and that the buds 

 will commence to expand ; Wieler was able indeed to induce bleeding 

 during the resting period without causing any recommencement of growth. 

 The external agencies by which an exudation of water may be excited do 

 not produce the same result in all plants and under all circumstances, as 

 indeed \ve might expect, since each plant has its own specific power of 

 reacting to a particular stimulus. A peculiar combination of conditions 

 occurring in nature may undoubtedly sometimes enable plants to bleed at 

 an unusual time. 



Wieler awakened a power of bleeding in various woody plants during 

 the resting period by decapitating them and subsequently repeatedly 



1 Wieler, 1. c., p. 107. 



