DAILF PERIODICITY OF GROWTH IN LENGTH. 823 



to this conclusion from observing that irregular variations of growth become less 

 the more the plant is protected from variations in the surrounding conditions. Partial 

 irregular neutralisations of the tension of the tissues may also cooperate to produce 

 this result. 



Sect. 18. — Periodicity of Growth in length caused by the alternation 

 of day and night. The alternation of day and night implies varying combina- 

 tions of the conditions of plant-life, especially of those that affect growth. Day 

 and night are distinguished not only by the presence and absence of sunshine, but 

 also by a consequent higher and lower temperature, which again causes variations 

 in the moisture of the air. Independently of special meteorological phenomena, 

 the temperature falls daily with the diminishing elevation of the sun till sunrise 

 the next day, that of the air rapidly, that of the ground more slowly ; at sunset the 

 fall is sudden, as is the rise at sunrise. In general the atmosphere approaches a 

 state of saturation as the temperature falls, z. e. the hygrometric difference decreases, 

 as it increases with the rising temperature. But these general daily alternations act 

 in a variety of ways, and even in opposite directions on the growth of plants ; the 

 increasing intensity of the light after sunrise retards growth, while the increasing 

 temperature promotes it, as long as the other conditions remain the same ; but the 

 increase of the hygrometric difference caused by the increasing temperature of the 

 air occasions also an increase of transpiration, which effects a diminution of the 

 turgidity of the tissues, and this again retards growth. 



It is important to ascertain which of these variable causes exercises the greatest 

 influence on growth ; and it will depend on this whether the growth of the plant 

 is most rapid by day or by night. On a cloudy but warm and damp day the weak 

 light has only a slightly retarding effect, but the temperature and the great amount 

 of moisture greatly promote growth ; under these circumstances the growth may be 

 greater than in the succeeding night (equal periods of time being compared), 

 when the total absence of light promotes growth, but the lower temperature is less 

 favourable to it. But the proportion may be reversed ; the plant may grow more 

 slowly by day than by night when the difference in the temperature and moisture 

 of the air during each is but small and very bright days intervene between dark 

 nights, the intense light retarding growth by day more than the depression of the 

 temperature by night. 



The greatest variety of combinations may be imagined in this respect; and from 

 the extreme changeableness of the weather the plant will, according to circumstances, 

 sometimes grow more quickly by day, sometimes by night, without exhibiting any 

 exactly recurrent periodicity. The numerous observations which have been made 

 in this direction do not therefore point to any general law^ It has however 

 been ascertained that, especially when long periods of time such as entire days are 



^ These will be found described by me in detail in the Arbeiten des bot. Inst, in WUrzburg, 

 1872, p. 170. [Baranetzky (Die fagliche Periodicitat im Langenwachsthum der Stengel, Mem. de 

 I'Acad. imp. de St. Petersbourg, XXVII, 1879) finds that there is a daily periodicity of the growth 

 of stems which is independent of the direct influence of any external conditions: (see also Bot. Zeitg. 



1877).] 



