282 THE MOVEMENTS OF WATER 



A plasmolytic excretion of water is still possible long after bleeding 

 and all active excretion of water have ceased owing to a diminution of 

 turgidity. If nectaries excreted water only in this latter manner, they 

 would dry up just when they were most likely to be useful to the plant, 

 for during the period of vegetative activity most plants are not sufficiently 

 turgid to be able to bleed. As it is the excretion of nectar may continue 

 however active transpiration may be, which is a fact of great biological 

 importance. Even when the plant is quite flaccid, fluid nectar may still 

 be present, and Wilson has shown that the feebly active nectaries on 

 the leaves of Prunns lanroccrasus may continue to excrete nectar if kept in 

 moist air, although the leafy branches may have lost more than one-fourth 

 of the amount of water normally present J . 



When transpiration is very active, the extracellular solution gradually 

 concentrates as the plant loses more and more water, and ultimately dries 

 up, as may readily be observed in many extrafloral nectaries. Indeed 

 the nectar is always more concentrated during periods of drought, and 

 crystals of sugar may even be found in it at such times, when it may 

 contain over 100 per cent, of sugar; the usual proportion is only 30 to 10 

 per cent., which may sink to less than i per cent, when the plant is fully 

 turgid and when no transpiration is allowed 2 . In this last case the amount 

 of fluid excreted is considerable, and drops of nectar may even overflow 

 and fall from the nectary. 



The nectary can apparently excrete as much water as is necessary 

 in this plasmolytic manner ; indeed under normal vegetative conditions 

 no active filtration is possible as a general rule, so that it is doubtful 

 whether the latter ever takes part in the process, for when the plant is 

 rich in water, any active filtration which might then be possible would 

 simply cause the plant to waste still more sugar by increasing the rate 

 at which nectar escapes from the overflowing glands. 



The absence of any active intracellular power of excreting water is 

 definitely proved in those cases in which the flow of water stops as 

 soon as all the extracellular sugar is removed by rinsing out the nectary 

 with water, and recommences when a little sugar is replaced upon them. 

 Instances are afforded by the floral nectaries of Fritillaria intperialis, Acer 

 pseudoplatanns, and the petiolar nectaries of Prnnus lanrocerasus (Wilson, 

 1. c.). The excretion of water by the nectaries cannot be stopped in this 

 manner if the plant itself is able to replace the sugar that has been washed 

 away, and in such cases the reappearance of the sugar shows that an 

 excretion of this substance is possible. The power of repeatedly excreting 



1 Unger, Flora, 1844, p. 707 ^ Nectaries on the petiole of Acacia) ; Wilson, Unters. a. d. Hot. Inst. 

 z. Tubingen, 1881, Bd. I, p. 8. 



a Sec Bonnier, 1. c., p. 85, and other works quoted later. 



