406 



TRANSFORMATION OF ENERGY 



Movements in fixed plants came under our notice in another relation, as for 

 example, when the root or the stem apex grows forwards in process of develop- 

 ment, and although such orthotropic movements are not those which, as a rule, 

 we have in mind when we speak of movements in plants, still it is impossible to 

 separate such movements as these from movements in general. The first type 

 of movement we naturally think of is the bending of an originally straight organ 

 or the alteration in curvature of an organ showing curvature to begin with. 

 We have now to study such curvatures both as regards their form and their 

 causes, and we may begin by considering such movements as appear in ripening 

 fruits or other drying parts, and whose cause lies in the loss of water from the 

 cell-membrane in other words, hygroscopic movements. 



The changes of shape which an organ thereby undergoes may be referred to 

 one or other of three fundamental types : thus we may speak of mere curvature, 

 when an originally straight organ becomes bent so that its axis lies in one plane ; 



of twisting when the axis of an 

 organ retains its original direc- 

 tion, while the longitudinal lines 

 of growth from being straight 

 become spirally twisted ; and, 

 finally, of twining, when the 

 entire organ becomes altered into 

 the form of a spiral. Fig. 119 

 shows these three conditions in 

 the case of a quadrangular prism, 

 and might also represent to some 

 extent similar changes of form in 

 the case of a stem. 



That the cause of the de- 

 formation in such organs lies in 

 the loss of water during desicca- 

 tion is shown by the fact that 

 they regain their original shape 

 when moistened, and that the one 

 or the other condition may be 

 induced at will according as water 

 is added or withdrawn. The 

 capacity for absorption of water is widely distributed in the vegetable kingdom 

 and may be due either to the osmotic activities of the cell-sap, or to the power 

 of imbibition possessed by the different parts of the cell. Hygroscopic move- 

 ments arise from the latter capacity, and especially from the imbibition of 

 water by the cell-wall ; indeed such movements take place even though the 

 parts under consideration consist of cell- walls only. We have several times re- 

 ferred to this phenomenon of imbibition, but now is the proper time to study it 

 in somewhat greater detail. 



We must first ask ourselves wherein lies the essential essence of the imbi- 

 bition. Bodies which are capable of swelling are able to absorb a liquid, and 

 thereby to increase in volume ; but it is obvious that this imbibition of liquid 

 must, to a certain extent, be limited, and that it brings about for the most part 

 an alteration in the consistent of the swollen body. In plants, water is the 

 medium which especially induces swelling, a medium which can also produce 

 swelling in other bodies which do not occur in plants. If we take a piece of 

 gelatine or glue, and determine its weight, and then lay it in water at an ordinary 

 summer temperature, we can observe an absorption of water as well as an in- 

 crease in volume, and with the aid of a balance we may convince ourselves that 

 the absorption takes place with decreasing rapidity and finally ceases. If the 



Fig. 119. Quadrangular prism 

 HI, twisted. IV, twined. 



/, straight. //, bent. 



