100 



PHYSIOLOGY OF NUTRITION. 



Fio. 70. -Stipa pennata. Fruit with awn. Two-thirds natural size. 



the awn is included between two bends, and carries the 

 plume of the awn. This plume is directed horizontally, and 

 if the fruit is submitted to the same conditions as the 

 Erodium fruit (see above), the absorption of water, by 

 causing the twisted part of the awn to unwind, makes the 

 fruit bore its way into the soil. The origin of these move- 

 ments in the fruits of Erodium and Stipa is to be sought 

 in certain structural characteristics of particular cells of 

 the spirally twisted portions of the fruits. 



1 See Detmer, Journal f. Landwirthsckaft, 27. Jahrgang, p. 110, and Kathay, 

 Sitzungsberichte der Akad. d. Wiss. zu Wien, Bd. 83. 



72. Absorption of Water by Fruits and Seeds. 



It frequently happens that juicy fruits (stone fruits, berries), 

 still connected with the parent plant, in rainy weather burst. 

 This is chiefly the result of the almost complete cessation under 

 such conditions of transpiration. The cells of the parenchyma of 

 the fruits turgesce very strongly, tensions are set up in the fruit 

 tissue (the epidermis in particular becomes violently stretched), 

 and finally the fruits burst. Another factor may in a subordinate 

 way co-operate in bringing about this result, viz. the absorption 

 of water by the fruits, whereby the turgidity of the parenchyma 

 cells is still further intensified. To prove that such absorption 

 actually takes place, uninjured cherries or grapes are accurately 

 weighed and laid in water with the fruit-stalks outside. After 

 four to eight hours they are taken out, carefully dried, and again 

 weighed. Generally it will be found that they have increased to 

 a not inconsiderable extent in weight. 



We lay a few wheat-grains in water. After about twelve hours 

 the process of soaking will already be far advanced, and the 

 grains will have become soft. The thin skin surrounding the 



