HYGROSCOPIC MOVEMENTS. 



399 



of position is well observed in the thin leaves of some mosses, the 

 grains generally (1) gathering on the side walls under bright light, 

 but (2) occupying the upper and lower faces of the cells when 

 the intensity of the light is much diminished. The first mode 

 of arrangement is termed apostrophe, the second epistrophe. 1 



1041. Hygroscopic movements are dependent upon the property 

 possessed by dry vegetable tissues of swelling more or less under 

 the influence of moisture. They are most strikingly exhibited in 

 the case of simple parts, like the filamentous appendages of the 

 spores of Equisetum and the teeth of the peristome of certain 

 mosses, notably that of Funaria hygrometrica. They are also 

 seen in the long appendages of 

 man} 7 fruits ; for example, in 

 the awns of some grasses, in 

 some Geraniaceae, etc., where 

 the}" serve the useful purpose of 

 fastening the fruit with its en- 

 closed seed in favorable soil. 

 When the fruit falls upon moist 

 soil, it at first lies flat; later, 

 the extremit}* of the appendage 

 and the tip of the fruit form 

 fixed points in the ground ; and 

 then, as moisture is absorbed 

 by the dry tissue, a spiral curva- 

 ture throughout the whole takes 

 place. This continues to twist 

 the tip of the fruit down into 

 the soil, much after the fashion 

 of a corkscrew. This kind of movement is most surprisingly 

 shown in some of the grasses of South America, and in our 

 native Stipa. 



In not a few instances the whole plant becomes relatively dry, 

 rolling up into a roundish mass which becomes expanded again 

 upon access of water. Good examples of such action are afforded 



1 In some cases the aggregation of the chlorophyll granules differs somewhat 

 from that described in the text. For a discussion of this subject, consult 

 Frank (Botanische Zeitung, 1871, and Pringsheim's Jahrbucher, viii., 1872), 

 also Stahl (Botanische Zeitung, 1880). Sachs, Prillieux, and Famintzin have 

 contributed much to the discussion. 



FIG. 176. Cross-section through the leaf of Lemna triscula, showing the position of 

 the chlorophyll granules: A, during the day ; B, during exposure to strong light ; C, 

 during the night. (Stahl.) 



