ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 685 



comparable to the water pouring out at the end of a hollow Archimedean 

 screw. 



Fruits. — The screw is beautifully illustrated by the fruits of the Stork's- 

 bill {Erodmm), the Feather-grass {Stipa), and the Oat. (Figs. 299, 300.) 



The first-named fruit consists of five carpels, separating from each 

 other when ripe, and having the long part above the ovary (really consist- 

 ing of the margins of the carpel) twisted. If it be moistened, the outer 

 side of the awn-like beak contracts, and the pseudo-awn then begins to 

 untwist and elongate itself. Since, too, the ovary and beak are both 

 covered with erect hairs, they act like anchors, and so give the fruit a 

 "purchase," with the aid of which it screws itself into the ground. 



We have seen how parts of flowers become subject to tensions during 

 growth, so that when complete they are in a condition of unstable 

 equilibrium, as the staminal tube of species of Medicago, which, when it 

 has sprung into its curved form, cannot be restored to its original horizon- 

 tal position. 



A similar tension may occur in fruits. Thus, in the Squirting 



Fig. 300. — Fruit of Feather-grass (Stipa pennata), showing spirally twisted 

 awn and sharp-pointed grain. (After Lubbock.) 



Cucumber the fruit is continually becoming more and more turgid with 

 juice as it increases in size. The tough rind prevents its bursting under 

 pressure, but when it is ripe the fruit gives way at the stalk and contrac- 

 tion immediately follows, so that the watery matter, with the seeds, is 

 squirted oat to a considerable distance. 



On the other hand, many dry fruits acquire a condition of unstable 

 equilibrium through the loss of water by evaporation. The tissue con- 

 tracts and the carpels burst open with violence, often ejecting the seeds to 

 considerable distances. Geraniums, Violets, Balsams, many of the 

 Euphorbia family, &c., discharge their seeds in this way ; but in the Wood- 

 sorrel it is the coat of the seed itself which, splitting and rolling back, 

 throws the seed out of the capsule. 



In order to resist the strain of the weight of a fruit as it increases in 

 size, the petiole becomes stronger and stronger till the fruit be ripe ; and 

 then the contrary takes place and the fruit falls. 



Attention may here be called to a curious result of the distribution of 

 forces and the consequent structure of many Pears (fig. 301). If, Like 



