492 MANUAL OF BOTANY 



contain, it is evident that this variation in activity is the ex- 

 pression of a form of irritability which it possesses. 



An instance of the stiinulating as contrasted with the tonic 

 influence of variation in temperature is seen in the opening and 

 closing of many flowers. Generally these are found to open 

 in the morning and to close at night, a behaviour which is very 

 similar to that of leaves showing nyctitropic movements. While 

 the leaves described above respond readily to variations in the 

 degree of illumination, flowers seem to be largely unmfluenced 

 by these, but to respond rather to the differences in the tem- 

 perature accompanying them. The variations, to be effective, 

 must lie, however, within the range already indicated as being 

 necessary for the manifestation of irritability at all. The 

 response made by the floral leaves seems, as in the case of the 

 foUage ones, to take the form of varying turgesoence of the 

 two sides of the growing part, followed in many cases by 

 actual growth. 



OHAPTEE XV. 



SPECIAL SENSITIVENESS AND ITS RESULTS. 



Besides the general reactions of protoplasm to variations in 

 those features of the environment which affect more or less the 

 whole plant, we find instances of special sensitiveness in various 

 parts to influences which are not appreciated by the whole of 

 the living substance. Of these the most prominent are lateral 

 light, gravity, contact with foreign bodies, and moisture. One 

 or two other cases of special sensitiveness affecting only par- 

 ticular organisms may also be discussed. 



Latbbal Light. — The effect of the lateral incidence of light 

 maybe studied very easily in the case of young seedlings. When 

 these are so placed that rays from a light-source illuminate one 

 side of their stem, very soon a curvature is apparent in the part 

 which is actively growing. This is of such a nature as to place 

 the axis of the plant in a position parallel to the incident rays. 

 It manifests itself sometimes very rapidly, at other times more 

 slowly. This response to a lateral stimulation is not confined 

 to the stems of seedlings, but may be seen to a greater or less 

 degree in many adult plants. It is a matter of common obser- 

 vation that geraniums grown in a window all bend their stems 

 towards the illuminated side. 



