334 HOW PLANTS LIVE AND WORK. 



Stamens are half-way down. Now, the thrum-eyed Primroses have 

 their stamens at the top, while the stigma of the pistil is half-way 

 down the tube, exactly opposite the part where, in the pin-eyed 

 form, the stamens are inserted. This curious state of things was 

 a great puzzle to botanists until Darwin cleared up the mystery. 

 A bee, thrusting its proboscis down a pin-eyed Primrose, would 

 dust it with pollen about half-way down, just in the place which 

 would come in contact with the pistil when the animal visited a 

 thrum-eyed flower. And the pollen from the thrum-eyed form 

 would adhere to the proboscis where this would touch the stigma 

 of a long-styled flower. This beautiful and simple arrangement 

 obviously ensures that each Primrose shall be fertilised by pollen 

 from the other form. 



The flowers of the Pea tribe — the Vetches, Clovers, and 

 Brooms — owe their quaint shapes to the visits of insects ; indeed, 

 it may be laid down as a general rule that all flowers with irregu- 

 lar corollas are fertilised by insects ; and the shape and colours of 

 corolla assumed depend upon the particular predilections of the 

 insect it is desired to entice. Bees and butterflies are very astute 

 critics both of form and colour, and flowers which in these 

 respects do not meet with their approval are left severely alone, to 

 " die unmarried, ere they can behold bright Phoebus in his 

 strength." The Labiates, for example, have their flowers so 

 modified that the lowest part of the corolla forms a platform, on 

 which the bee may conveniently alight, while the upper petals 

 unite into an arched roof, which protects the pistil and stamens. 

 This order is particularly interesting in its relations to insects, and 

 no better example could be taken than that of the Sage. This 

 flower contains four stamens, but two of these have lost their use, 

 and the others are modified in a strange manner. The whole 

 stamen has somewhat the shape of a capital T, and at each end of 

 the cross-piece is a pollen-box. Usually the cross-piece is not at 

 right angles to the stalk, but is swung up (the junction acts like a 

 hinge) until it is nearly vertical. The pollen-box which is at the 

 lower end of the cross-piece when this is vertical contains hardly 

 any pollen. Thus we get the entrance to the honey tube guarded 

 by two pillars, the stalks of the stamens ; and the lower pollen-box 

 of the cross-piece of each stamen is directly in front of the bee's 



