262 



TAKING UP OF POLLEN BY INSECTS. 



of cases the filaments are irritable and on the slightest touch undergo a change 

 of position analogous to the closing up of the leaves of the Sensitive Plant 

 (Mimosa; see vol. i. p. 537) vi^hen subjected to a like stimulus. 



The best known examples of the hammer form of mechanism occur in the 

 genus Salvia. In no species of that large genus is it developed to greater perfec- 

 tion than in Salvia glutinosa, which we therefore select for illustration. The 

 flowers of this Labiate are set laterally on the stem, and the under-lip serves as 

 landing-stage for the humble-bees to alight on (see fig. 271). If a bee, after alight- 

 ing, is to obtain the honey which is hidden in the back part of the flower near 



Fig. 271. — transference of pollen to the bodies of insects oy means of mechanism of the percussive type. 



1 Part of an inflorescence of Salvia glutinosa, ; the right-hand flower is being visited by a humble-bee, and the pollen-covered 

 anther is in the act of striking the insect's back. 2 Another part of the same inflorescence with three open flowers in 

 different stages of development : the lower flower on the left-hand side is being visited by a humble-bee which carries 

 on its back pollen from a younger flower and is rubbing it off on to the deflexed stigma, s A stamen of Salvia glutinosa 

 with rocking connective. * Longitudinal section through a flower of the same plant. The arrow indicates the direction 

 in which humble-bees advance towards the interior of the flower. 5 Same section ; the lower arm of the connective-lever 

 is pushed backward, and in consequence the pollen-covered anther at the end of the other arm of the lever is deflexed. 



the ovary, it must advance from the under-lip into the gaping jaws of the flower. 

 It is there that the curious mechanism is set ready. On either side of the entrance 

 is a stamen (see fig. 271 ^) composed of a short, erect, firm, and immovable filament, 

 and an anther borne at the extremity of a much elongated and slightly curved 

 connective which rocks at the top of the filament. The connective is articulated to 

 the filament in such a manner as only to be able to swing in the direction indicated 

 in figs. 271 * and 271 \ The part of the stamen which is liable to be set rocking 

 is a curved lever consisting of two arms of unequal length. The upper arm is the 

 longer and terminates in the anther, whilst the under and shorter arm is somewhat 

 thickened and spatulate at its free extremity. When a humble-bee pushes the 

 lower arm in the direction of the arrow (fig. 271 *) the upper arm drops (fig. 271^). 



