ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 



688 



for striking the bee just where the pollen has been deposited from a 

 previously visited flower. 



A lever of the second kind is not uncommon among plants ; a familiar 

 illustration is seen in a sugar chopper, in which the knife is hinged at 

 one end, the handle being free to move up and down at the other ; while 

 the block of sugar is between the fulcrum at the hinge and the iwwer 

 in the hand, and constitutes the weight, i.e. in this case, the resistance 

 to the chopper. A pair of nutcrackers is a double lever of the same 

 kind ; the nut gives the resistance to be overcome between the common 

 ''fulcrum " at the hinge and the two "powers " in the hand. 



A remarkable illustration of this kind of lever occurs in the develop- 

 ment of the stamens of the flower of the Strawberry tree {Arbutus Unedo). 

 In the bud the anther possesses a pointed tip, which is in contact with the 

 style and fixed to it by a gummy secretion (fig. 297 a). The anther is capable 

 of moving by a sort of joint at its base where it is attached to the filament. 

 Consequently, as the latter elongates, the lower ends of the anther-cells 

 become tilted up, as if to rotate round the fixed point. This motion 

 continues until the anther is quite inverted (b). Then the cells dehisce 

 at the highest point, this being really the true base of the cells. 



Fig. 298. — Diagram to illustrate the distribution of forces 

 in the growing stamens of Strawberry tree. 



In this process (illustrated by figure 298) the fulcrum (f) is the point 

 of adherence at the apex. The weight is the resisting body of the anther 

 (w) ; while the ijoicer (p) is the vegetative force of growth trying to 

 elevate the anther, which it simply overturns by rotating it round the 

 pivot as a fulcrum. 



Lever of the third kind. In this the power is between the fulcrum 

 and the weight. Thus, in holding out a heavy weight at arm's length, 

 the fulcrum is the elbow-joint, the power resides in the muscles of the 

 fore -arm. 



Now, a large number of flowers have no front petal upon which an 

 insect can alight. In such cases the stamens, and sometimes the style as 

 well, first curve downwards and then again upwards, as already described in 

 speaking of the leverage of a horizontal bough of a tree. The point of 

 attachment of the filaments to the corolla or floral receptacle is the 

 fulcrum ; the weight of the insect is at the further end, or near it ; and 

 the power exerted by the filaments is situated at the curve near the base 

 of attachment ; so that the filaments are like a horizontal reversed Ji . 

 The arrangement will be understood from figure 279. 



The Lever and a Hollow Screw. — In the Scarlet Runner there is a 

 combination of a lever with what somewhat resembles an Archimedean 

 screw used for elevating water. In this flower the keel petals, instead of 



