Brown. — Studies in the Physiology of Parasitism. VIII. 105 
treated petals. There is thus a distinct ageing effect (as was to be expected). 
The wetting effect is readily observable. It was seen that the earlier drops 
laid on a given spot were, on application of the pipette, readily sucked up in 
their entirety, whereas in the case of the later drops a film of liquid was 
left on the surface of the petal. Whenever sirch a degree of wetting was 
reached, i. e. such that it was impossible to pick up the complete drop with 
the pipette, high exosmosis figures were invariably obtained. 
The presence of pollen-grains in the drops gives rise to high conductivity 
figures. This is shown by the following expfiHment with Cereus petals. 
The petals were washed (without rubbing) in a stream of water, rinsed with 
distilled water, then allowed to dry, after which drops of distilled water were 
placed on each. To a certain number of these a quantity of Cereus pollen 
was added on the point of a platinum wire. After twenty-four hours the 
conductivity of the drops was measured, with the following results : — 
Pollen-free drops. Average conductivity after 24 hours V n drops) = 3-3 
(min. 2.8, max. 4-2) 
Pollen-containing drops. Average conductivity after 24 hours (6 drops) = 73-8 
(min. 54.6, max. 92.3) 
It should be stated that the pollen-containing drops show a greater 
tendency to spread over the surface of the petal than the pollen-free ones. 
This in itself, as giving rise to a greater area of contact with the petal, 
would tend to cause the conductivity figures to be higher in the former 
case. The disparity in this respect, however, would account for only a very 
small portion of the difference in conductivity observed. Similar results 
were obtained with pollen of Rose and Lily, and it is highly probable 
that they apply to pollen generally. 
In determining the amount of exosmosis from the petal itself it is 
therefore necessary to remove completely any pollen that may be present. 
This can be effectively done in the manner described above. 
Incidental to this necessity for the removal of pollen, it was found that 
mechanical rubbing, even though gentle, considerably increased the con- 
ductivity figures obtained. Thus in one experiment with petals of Cereus 
the average conductivity of drops laid along the midrib of washed and 
slightly rubbed petals was J2-8, whereas that of drops laid on a similar 
series of petals (alternating in the flower with the preceding) which had 
simply been rinsed amounted to only 5-0 (19 drops in each case). In the 
case of the rubbed petals it was observable that better contact between the 
petal and the drop had been established. The effect is undoubtedly due 
to the removal of the surface bloom of the petals by the mechanical 
treatment. 
Throughout this work strict attention was paid to the question as to 
whether the experimental petals remained alive during the treatment. In 
petals generally, and in Cereus in particular, death is shown macroscopically 
