DAILY PERIODICITV OF GROWTH IN LENGTH. 
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. i8. — 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-hfe, 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, i. 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 hght 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 exhibidng 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 Wür^urg, 
1872, p. 170. [Baranetzky (Die tägliche Periodicität im Langen wachsthum der Stengel, Mem. de 
l'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).] 
