522 BOTANY 



small knob at the end of the long connective, which is balanced upon 

 the short filament so that it oscillates readily when the lower end is 

 touched. This lower end is placed near the entrance of the nectary, 

 and a bee, probing the throat of the flower, pushes against the lower 

 end of the connective, whose upper, pollen-bearing end is forced 

 down with a spring against the back of the bee, upon which it depos- 

 its the pollen, resuming its original position when the bee backs out 

 of the flower. At this time the pistil is relatively short, and out of 

 reach of the insect's body ; but if the insect visits an older flower, 

 the pistil hangs down, so that the stigma is directly in the way of 

 its back, against which it in turn strikes, and receives the pollen 

 brought from a younger flower. * If insects are excluded from the 

 flowers, pollination is impossible. 



The various species of Iris offer excellent examples of strictly 

 entomophilous flowers. In these the stamens are three in number, 

 and quite concealed by the overarching petaloid styles, against 

 which they are closely pressed (Fig. 492). The stigma forms a little 

 shelflike outgrowth above the stamens, but quite out of reach of any 

 pollen which might be accidentally shed from them, and also having 

 the receptive surface turned outward so that an insect drawing away 

 from the flower, after having carried off the pollen, does not leave 

 any pollen upon the. stigma. If, however, it visits another flower, as 

 it crowds between the petal and the style, the edge of the stigma 

 scrapes off any pollen that may be attached to it. 



Sensitive Parts. In a number of flowers certain parts are sensi- 

 tive to touch, and these are mostly concerned with the question of 

 pollination. One of the simplest cases is seen in the stamens of 

 various species of Berberis. These, when touched near the base, as 

 happens when a bee is probing for honey, Avill spring violently in- 

 ward, shaking off the pollen, and scattering it upon the insect visitor. 

 Somewhat similar is the case of Kalmia, where the stamens are bent 

 over into little pockets, from which they spring out when touched, 

 throwing the pollen to some distance. In many Leguminosae e.g. 

 species of Cytisus (Fig. 489) the pollen is deposited in the pouch 

 formed by the coherent keel-petals. When a bee lights upon the 

 flower, these are pushed down, and the stamens are liberated sud- 

 denly, sending out the mass of pollen in a little cloud, which dusts 

 the body of the insect. At the same time the stigma is exposed, and 

 is ready to be pollinated by the next bee which visits the flower. 



In the Milkweed family there are a number of devices for insuring 

 the transference of the pollen to the stigma, but these sometimes 

 result disastrously to the insect. The pollen in the common Milk- 

 weed (Asclepias) is in little pear-shaped masses or pollinia, which 

 are united in pairs, and must be withdrawn from the anthers, either 

 by the proboscis of a butterfly or bee, or sometimes by the legs of 



