332 



BOTANY 



PART I 



on this account these organisms have been used to demonstrate the 

 assimilation of carbon dioxide (p. 248). 



The phenomenon of hydrotaxis, a directive movement due to the 

 unequal distribution of water -vapour in the air, may be associated 

 with chemotaxis. A positive hydrotaxis is shown by the plasmodia of 

 Myxomycetes, and this passes into negative hydrotaxis at the time of 

 spore-formation. 



II. MOVEMENTS OF CURVATURE 



The kinds of curvature which may take place in the organs of 

 attached plants are illustrated by Fig. 274. A four-angled prism 



is of equal length along 

 each of its angles. If it is 

 bent in one plane the angles 

 of the concave side must 

 become markedly shorter 

 than those of the convex 

 side. An elongation of 

 one side or a shortening of 

 the other side or simultane- 

 ous lengthening of one side 

 and shortening of the oppo- 

 site side must lead to curva- 

 ture. When in this process 

 of bending the column 

 remains in one plane, it is 

 spoken of simply as curved. 

 When, however, it passes 

 out of the one plane so 

 that the bending follows a 

 line oblique to the longitu- 

 dinal axis it is spirally wound (IV). Lastly, when the column remains 

 as a whole straight but its angles follow spiral lines, it is termed 

 twisted (III). The torsion comes about by a difference in length be- 

 tween the middle line and the angles ; all the latter are of equal length. 

 Ways in which Curvatures are produced. In the production of 

 curvatures we are always concerned, as has just been shown, with 

 changes in the dimensions of an organ due to unequal lengthening or 

 shortening. In bringing about these changes in dimension the follow- 

 ing means are employed by the plant. 



1. Growth. This can only lead to elongation. 



2. Osmotic pressure. This can effect an elongation or a shortening 



according as it is increased or diminished. 



Variations in the amount of water in the cell wall or in dead 

 cells. These also can effect either elongation or shortening. 



FIG. 274. Four-angled prism. I, Straight; //, curved 

 III, twisted ; IV, spirally wound. 



3. 



