MESSMATES, MUTUALISTS, AND PARASITES 91 



are crowded together. The most specialized plants lay themselves 

 out, so to speak, to attract insects with well-marked powers of 

 flight, or in some cases birds, and it may be mammals. These are 

 the " bidden guests". But there are also " unbidden guests" with 

 a liking for nectar, especially wingless insects, such as ants. In 

 many cases these undesirable visitors simply rob the flowers with- 

 out conferring any benefit. There are also numerous snails and 

 slugs which eat up flowers altogether if they get the chance. It is 

 for all reasons desirable that these useless and dangerous visitors 

 should be kept away. Ogle, in the preface to his translation of 

 a work by Kerner (Unbidden Guests), thus presents the matter: 

 " Now Nature, who at first sight often appears a prodigal, is 

 always found, on closer examination, to be the most rigid of econo- 

 mists. If no insects are to be allured, she gives ... no nectar; 

 she cuts off the bright petals and suppresses the attractive odours. 

 Nor even when a bait is wanted will she give it one minute sooner 

 than is necessary. The brilliancy, the scent, and the nectar are only 

 furnished when the flower is ready for its guests and requires their 

 presence ; just as a thrifty housewife lights her candles when the first 

 guest is at the door. The mature bud is furnished with no such 

 attractions. Still more, even when the flower is mature, when its 

 pollen is ready for transference or its stigma for pollination, when all 

 the allurements are consequently displayed and insects invited to the 

 feast, she still shows her economy. Guests might come who were 

 not of sufficient importance, and the banquet be wasted on them; 

 for it is only when insects have a certain shape, size, or weight 

 that she requires their visits, and can use them profitably for her 

 purposes. She requires, moreover, that they should make their 

 entrance by the main portal, which she has specially adapted to 

 suit their and her requirements. All insignificant and unremunera- 

 tive visitors, all such, moreover, as would creep in by the back 

 entrance, must be kept out. ..." 



It will be convenient to first describe the chief ways by which 

 wingless insects are kept away from the flowers. One rather 

 curious device is found in some of the Balsams, e.g. in a species 

 native to the Himalayan region (Impatiens tricornis) which has 

 been carefully studied. Here, as in many other plants, there are 

 little expansions (stipules) at the bases of the leaves, where they 

 join the stem. Of each pair of stipules one is transformed into a 

 nectar-secreting gland (nectary) in the form of a thick curved plate, 



