228 BOTANY. 



disc hardening like cement on exposure to the air. As the 

 insect withdraws its tongue, one or both of the pollen masses 

 are dragged out and carried away. The action of the insect 

 may be imitated by thrusting a small grass-stalk or some simi- 

 lar body into the spur of the flower, when on withdrawing it, 

 the two pollen masses will be removed from the flower. If 

 we now examine these carefully, we shall see that they change 

 position, being nearly upright at first, but quickly bending 

 downward and forward (Fig. 89, D n, m), so that on thrusting 

 the stem into another flower the pollen masses strike against 

 the sticky stigmatic surfaces, and a part of the pollen is left 

 adhering to them. 



The last arrangement that will be mentioned here is one dis- 

 covered by Darwin in a number of very widely separated plants, 

 and to which he gave the name "heterostylism." Examples 

 of this are the primroses (Primula), loosestrife (Lythrum), 

 partridge-berry (Mitchella), pickerel-weed (Pontederia) (Fig. 

 84, /), and others. In these there are two, sometimes three, 

 sets of flowers differing very much in the relative lengths 

 of stamens and pistil, those with long pistils having short 

 stamens and 'vice versa. When an insect visits a flower with 

 short stamens, that part is covered with pollen which in the 

 short-styled (but long-stamened) flower will strike the stigma, 

 as the pistil in one flower is almost exactly of the length of 

 the stamens in the other form. In such flowers as have three 

 forms, e.g. Pontederia, each flower has two different lengths 

 of stamens, both differing from the style of the same flower. 

 Microscopic examination has shown that there is great varia- 

 tion in the size of the pollen spores in these plants, the large 

 pollen from the long stamens being adapted to the long style 

 of the proper flower. 



It will be found that the character of the color of the flower 

 is related to the insects visiting it. Brilliantly colored flowers 

 are usually visited by butterflies, bees, and similar day-flying 

 insects. Flowers opening at night are usually white or pale 



