IRRITABILITY. 405 



then, of variations of external conditions is more or less 

 marked in proportion as it coincides with or opposes the 

 course of the daily period. 



A few remarks may be added with regard to the rising 

 and falling of growing foliage-leaves (see p. 400). It has long 

 been known that the young growing leaves of various plants 

 take up different positions by day and by night. Batalin, 

 who has the most recently studied this subject, mentions the 

 following cases : leaves which rise at night, Chenopodium, 

 Polygonum aviculare, Stellaria, Linum : leaves which fall at 

 night, various species of Impatiens, Polygonum Convolvulus, 

 Sida Napcea. Batalin has shewn that these movements, like 

 the opening and closing of flowers are undoubtedly phe- 

 nomena of growth, and probably their daily periodicity is 

 induced in the same way in both these case's. 



Daily periodicity of the Tensions of the Tissues. In a 

 previous lecture (p. 342) attention was drawn to the existence 

 of tissue-tensions in growing organs, and the mechanical con- 

 ditions upon which these tensions depend were discussed. 

 We will now study them in their relations to external condi- 

 tions, and we shall find they afford information which will 

 materially contribute to an understanding of the mode in 

 which variations in the external conditions give rise to cor- 

 responding variations in the rate of growth. 



In the course of his researches, to which allusion was then 

 frequently made, Kraus ascertained that the tissue-tensions 

 are considerably affected by variations in the external condi- 

 tions. He observed that the tensions, both longitudinal and 

 transverse, exhibit a daily periodicity in shoots which are 

 exposed to the normal alternation of day and night, such 

 that the tensions diminish from dawn throughout the day as 

 the intensity of light increases, and begin to increase in the 

 early afternoon as the intensity of the light diminishes. This 

 periodicity is dependent upon previous exposure to the alter- 

 nation of day and night, for if a shoot be kept in continuous 

 darkness, the daily period gradually disappears, and is 

 replaced by irregular variations. The daily period can only 

 be observed when the temperature is sufficiently high ; thus, 



