86 



ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



Such flowers open in the evening, and are then most fragrant. 

 Conspicuousness is often increased by numerous flowers being 

 associated together. 



For the entertainment of their insect-guests flowers may pro- 

 vide sweet sap, or produce a great excess of pollen, a highly 

 nutritious substance, or secrete nectar. In highly-specialized 

 forms at least the last sort of food is generally the most important, 

 and is usually produced deep down in the. recesses of the flower, 



often in long spur-like tubes, 

 where it is only accessible to 

 long - tongued insects, such 

 as bees and butterflies. 



Flowers not only attract 

 guests and provide refresh- 

 ment, but their whole struc- 

 ture is often modified, it may 

 be in a very complex way, 

 to secure the benefits of 

 cross-pollination, i.e. to en- 

 sure that pollen which is 

 brought is deposited on the 

 stigma by arriving guests, 

 and make certain that de- 

 parting guests take with 

 them a fresh supply of the 

 same material. 



Night, a flower being visited by a moth (Dianthacia albi- ^ manV CaSCS the adap- 



macula}; the remains of dead creeping insects are seen adher- tationS CxistinP" between 

 ing to the viscid stem ^ 



flower and insect are so 



perfect as to leave no room for doubt that each has influenced 

 the evolution of the other. Some of the adjustments that have 

 come into existence will best be understood by reference to con- 

 crete examples, the first two of which will be taken from the 

 remarkable Orchis Family, the members of this being notable for 

 the great variety of arrangements which they display in relation to 

 insect-pollination. In one large and handsome species (Phalcz- 

 nopsis Schilleriana, fig. 1080) the attraction of colour is provided by 

 five spreading leaves, equivalent to sepals and petals, of which the 

 most remarkable (the labellum) is one which hangs down from 

 the centre of the flower. It begins in a narrow stalk, but soon 



by 



