52 {ISOLATION BY WATER. 
bird, but also the contrivances for preventing insects 
that would not be useful from obtaining access to the 
nectar. Thus the immense length of the Angracum 
sesquipedale of Madagascar might, perhaps, have been 
more easily explained by Mr. Wallace, if this impor- 
tant purpose had been taken into account.’ 
Kerner has since published a very interesting 
work,! especially devoted to the subject, which has 
been translated into English by Dr. Ogle. 
In aquatic plants, of course, the access of ants is 
precluded by the isolation in water. Nay, even many 
land plants have secured to themselves the same advan- 
tage, the leaves forming a cup round the stem. Some 
species have such a leaf-cup at each joint, in others 
there is only a single basin, formed by the rosette of 
radical leaves. In these receptacles rain and dew 
not only collect, but are retained for a considerable 
time. In our own country Dipsacus sylvestris (the 
common teazle) is the best marked instance of this 
mode of protection, though it is possible that these 
cups serve another purpose, and form, as suggested by 
Francis Darwin, traps in which insects are caught, and 
in which they are dissolved by the contained fluid, so 
as to serve as food for the plant. However this may 
be, the basins are generally found to contain water, 
even if no rain has fallen for some days, and must, 
therefore, serve to prevent the access of ants. 
‘The next mode of protection is by means of slippery 
' Kerner: Flowers and their Unbidden Guests. 
