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 Angrcecum 

 sesqiiipedale 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 Unliddeii Guests. 



