CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 745 



light is the result of the absence of the retarding paratonic action 

 of light. 



The exceptions to this rule are the shoots of stem-climbers (Hop, etc.). The 

 stems of these plants have, under ordinary conditions, very long internodes ; 

 they seem to grow to the utmost even although they are exposed to light, and 

 they do not grow to any greater length in darkness. On account of this pecu- 

 liarity they have been termed " normally etiolated shoots." 



The effect of the paratonic action of light can also be estimated 

 by direct measurement of the growing member. As the result of 

 a great number of comparative measurements, it has been found, 

 in regard to members of all kinds, that the rate of growth is more 

 rapid in darkness than in light. 



An interesting demonstration of the relation of the rate of 

 growth to light, is afforded by the observation of the growth of 

 any member at given intervals every hour, or every two or three 

 hours during an entire day of twenty-four hours. By this means 

 it has been ascertained that a growing member exhibits a regular 

 daily periodicity in the variations in its rate of growth, which has 

 a direct relation to the alternation of day and night. 



A general inspection of the tracings in Fig. 480 shows that the 

 rate of growth increases in darkness, and diminishes in the light. 

 This is more apparent in the 3-hour than in the 1-hour tracing, 

 for the latter is complicated by the registration of the spontaneous 

 variations in the rate of growth (see p. 739), whereas the former is 

 not, owing to the longer interval between the measurements. 



It will be further noticed that the effect of a change from dark- 

 ness to light, or from light to darkness, is not manifested at once ; 

 but that after such a change, the rate of growth goes on, for a, 

 time, as it did before, until gradually it alters. In fact, a growing 

 member, though kept in continuous darkness, will exhibit a more 

 or less well-marked periodicity for several days. This is a good 

 instance of the general slowness of reaction which characterises 

 growing members. 



But it must be borne in mind that variations of temperature 

 also affect the rate of growth. Hence, since the alternation of 

 day and night is accompanied by changes of temperature, the 

 forms of the tracings in the figure are not solely due to the influ- 

 ence of light, but also to some extent to that of temperature. 

 Speaking generally, it may be said that, as the intensity of the 

 light diminishes towards evening, the temperature falls. The 



