ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 679 



of flowers in connection with insect fertilisation, there are numerous 

 instances to be seen of the use of mechanical forces for executing that 

 important function. 



There is, first, the actual building up of the calyx and corolla with 

 strengthening ribs, &c., so as to be able to sustain the weight of the 

 insects without collapsing under it. This is well seen in tubular flowers 

 of many of the Gamopetalous division. If the corolla is a strong one, 

 and able of itself to support the insect, as of the Foxglove, into the long 

 tubular corolla of which a large humble-bee can crawl, there is no 

 necessity for any additional aid from the calyx, which remains polysepalous. 

 Bat if the tube of the corolla be small and slender in comparison with 

 the limb, which may consist of a large lip in front and a hood behind, 

 then the calyx may undertake to support the corolla-tube, so that it will 

 not yield to the weight of a large insect. This is well seen in many of the 

 Labiates,* as in the Dead-nettle. Thus, in Salvias, the calyx is some- 

 what two-lobed, as if it were "stretched," with a tendency to split at the 

 sides. To avoid doing this, and to strengthen the calyx-tube, several 



Fig. 292. — Lucerne (Medicago sativa): 

 a, flower, seen from front, ready 

 for insects ; b, after explosion ; c, 

 corolla removed, to show the rigidly 

 curved position of the stamens. 



Fig, 293. — Male flower of Stinging- 

 nettle, with expanded sepals and 

 spread-out stamens. 



c 



c 



c 



c 



c 



* 



additional woody cords are introduced along the lines of junction of th& 

 sepals. There are more in the anterior than in the posterior half, as the 



strain is greatest in front. If C in the accom- 

 panying diagram represent the normal cords or 

 midribs of the calyx-sepals, the asterisks will stand 

 ^ for additional ones along their united margins ; 

 ^ the uppermost C being on the posterior side of the 

 flower. Corolla-tubes are often strenofthened bv 

 having numerous fine cords running up them in 

 * addition to the primary five, which characterise the 



five petals, if it be a Dicotyledon. 

 Springs. — Besides these strengthening arrangements by means of extra 

 cords, strong folds often occur, and these may act as movable springs and 

 levers. In some leguminous plants very strong springs occur, as in 

 Genistas (fig. 291) and Medicago (fig. 292). In the former the claws of the 

 wing and keel petals are at first horizontal (fig. 291 a), but by the time the 



* Both the shape of the calyx and its strong ribs will be seen to be in adaptation 

 to the corolla in order to support it. 



