IRRITABILITY. 403 



transpiration. Similarly, the rise of the curve of growth 

 during a period of darkness is not so rapid as it might be, 

 because the accompanying low temperature exercises a slight 

 retarding influence ; but, on the other hand, the acceleration 

 may be promoted by the diminution of transpiration. 



A similar daily periodicity has been observed by Prantl 

 in the growth both in breadth and length of the leaves of 

 dicotyledonous plants, by Stebler in the growth in length of 

 the leaves of monocotyledonous plants, and by Strehl in the 

 growth in length of roots. 



The daily periodicity of growth induced in a plant by 

 exposure to the normal alternation of day and night does not 

 necessarily cease when the plant is kept in continuous dark- 

 ness, but may persist for a longer or shorter time. Baranetzky 

 has observed this in various species of Gesneria, in Helianthus 

 annuus and tuberosus, and in Brassica Rapa. But the duration 

 of this persistence varies in different plants. Baranetzky could 

 observe the daily period in Gesneria tubiflora for only two or 

 three days, whereas he could detect it in Helianthus tuberosus 

 after fourteen days in continuous darkness. The occurrence of 

 the minima and maxima becomes eventually irregular, and is 

 then the expression of the irregular spontaneous variations of 

 growth (see p. 358). Entirely etiolated plants, plants, that is, 

 which have been grown from the beginning in darkness and 

 have not been exposed to the alternation of day and night, do 

 not exhibit a daily periodicity of growth, but only the irregular 

 spontaneous variations. It was indeed mentioned in a pre- 

 vious lecture (p. 359) that Baranetzky observed a fairly regular 

 daily period in the growth of the stem of an entirely etiolated 

 plant of Brassica Rapa. He is inclined to regard this as due to 

 heredity, but it is at least equally probable that it is a mere 

 coincidence, that the spontaneous variation happened in this 

 case to occupy a period of twenty-four hours. 



Some interesting examples of daily periodicity of growth 

 are afforded by the opening and closing of the flowers, and by 

 the rising and falling of the leaves, of certain plants. With 

 regard to the opening and closing of flowers, we know 

 already that opening is induced by exposure to light, and 



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