;>Q4 PLANT STRUCTUKKS 



rapidity. In these cases the part concerned develops a 

 curvature, and by various curvatures it attains its ultimate 

 position. These curvatures are not necessarily permanent, 

 for a perfectly straight stem results from a series of cur- 

 vatures near its apex. Curvatures may be developed by 

 unequal growth on the two sides of an organ, or by unequal 

 turgidity of the cells of the two sides, or by the unequal 

 power of the cell walls to absorb water. 



169. Hygroscopic movements. These movements are only 

 exhibited by dry tissues, and hence are not the direct result 

 of the activity of protoplasm. The dry walls absorb mois- 

 ture and swell up, and if this absorption of moisture and 

 its evaporation is unequal on two sides of an organ a curva- 

 ture will result. In this way many seed vessels are rup- 

 tured, the sporangia of ferns are opened, the operculum of 

 mosses is lifted off by the peristome, the hair-like pappus 

 of certain Composites is spread or collapsed, certain seeds 

 are dispersed and buried, etc. One of the peculiarities of 

 this hygroscopic power of certain cells is that the result 

 may be obtained through the absorption of the moisture of 

 the air, and the hygroscopic awns of certain fruits have 

 been used in the manufacture of rough hygrometers 

 (" measures of moisture "). 



170. Growth movements. Growth itself is a great physi- 

 ological subject, but certain movements which accompany 

 it are referred to here. Two kinds of growth movements 

 are apparent. 



One may be called nutation, by which is meant that the 

 growing tip of an organ does not advance in a straight 

 line, but bends now toward one side, now toward the other. 

 In this way the tip describes a curve, which may be a 

 circle, or an ellipse of varying breadth ; but as the tip is 

 advancing all the time, the real curve described is a spiral 

 with circular or elliptical cross-section. The sweep of a 

 young hop-vine in search of support, or of various tendrils, 

 may be taken as extreme illustrations, but in most cases 



