DAILY PERIODICITY OF GROWTH IN LENGTH. 743 



from one to three hours or more. These phenomena I call irregular variations 

 of growth \ They appear to result from the plant being subjected to continual 

 small variations of temperature, air, light, and moisture of the soil, which alter the 

 turgidity, and therefore the extensibility and elasticity of the growing cells. I come 

 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, i 8.— 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, /. 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 uncertain which of these variable causes may have 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 spaces 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 



^ For further details see Reinke, Verhandl. des bot. Vereins fiir die Provinz Brandenburg, 

 Jahrg. VII; and Sachs, Arbeit, des bot. Inst, in Wiirzburg, Heft II, p. 103. 



2 These will be found described by me in detail in the Arbeiten des bot, Inst, in Wiirzburg, 

 1872, p. 170. 



