f^T"^"-^ 



THE EFFECTS OF STRAINS ON STRUCTURES. 125 



by arising in the axil of a horizontally inserted leaf), much 

 more often than in a strictly horizontal direction. The 

 branch, after growing for a short distance upwards, generally 

 bends downwards, assuming just the same curvature as of 

 declinate stamens which have to support the weight of 

 insects. 



If the vertical line in the adjoining diagram (Fig. 39) 

 represent the trunk, and the curved 

 line a branch, the insertion at /sup- 

 plies the'fulcrum, w is the weight of /I 

 the branch, and acts in a vertical I \vA 

 line, p is the power required to 

 counteract the resultant of these two 

 forces. ^ 



Whpn fhp Tinncrli Lrpalc«? pU>,p-p Fig. 39.— Diagram of a tree and 



vv nen rne oougn oreaKs, eitnei ij^anch, illustrating the distri- 

 through an additional weight of snow ^'^^^^^ offerees. 

 or by its own weight on decay, it snaps off at the point p, 

 i.e. the place where the force acts, as it can no longer over- 

 come the resultant of / and w. 



Reproductive Organs. — Applying these principles to 

 floral structures, we have already seen in how many Avays 

 the strain to which parts of flowers are subjected, through 

 the weights and pressures of insects, are met and overcome. 



In a large number of instances the organ becomes curved, 

 and assumes the character of a spring, yielding on pressure, 

 but recovering its position when pressure is removed. It is 

 often so with the claws of the petals of papilionaceous 

 flowers, the stamens of Dicenira, CorydaUs, and Veronica 

 Chamcedrys. Similar structures are seen in many styles, as 

 those of Pansy (Fig. 54), and in genera of Polygalacece. 



All declinate stamens partake of it to a more or less 

 degree. The distribution of the forces brought into play to 

 support the insect is exactly the same as when a bough 



