554 LECTURE XXXII. 



means, however, the turgescence of the plant may be altered ; for instance, if the soil 

 is very warm thC' roots take up much water and the turgescence is increased, if the 

 temperature of the air does not suffice to cause active transpiration (such is the case 

 for example in the evening after a warm day) ; on the contrary, the turgescence is 

 diminished if, the temperature of the soil being low, the roots take up water slowly, 

 while a warm wind or sunshine incites the leaves to transpire strongly (as, for example, 

 after sunrise after a cold night). By the alterations of turgescence thus brought about, 

 however, the recorded rapidity of growth will be affected at the same time. During 

 observations in the open, also, these conditions may distort the result as to the effect 

 of temperature, which is being investigated, until it is unrecognisable, and thus observa- 

 tion in the laboratory again recommends itself, where the air is quiescent, and very 

 slow and slight variations of temperature take place, to which the earth in the flower- 

 pot can adapt itself : although under such conditions the temperature of the latter is 

 usually several degrees lower than that of the air, the difference is nevertheless small 

 and almost constant — i. e. the temperature of the air and of the soil (in the pot) it 

 represented as curves almost coincide.' 



A third category of influences affecting the daily course of growth occurs in the 

 alternation of light and darkness — in the decrease and increase in the intensity of the 

 light. ' Unfortunately,' I said, ' we have as yet no convenient method of so measuring 

 the varying intensities of light that the measurements are directly applicable to the ob- 

 servation of plants: measurements of the brightness perceptible by the eye, even if they 

 - could be conveniently determined, would present something other than the required 

 measurement of those rays of light which influence growth in length; for these, as direct 

 observation and the heliotropism occurring in coloured light show, are the blue, violet, 

 and ultra-violet, that is, the inappropriately so-called chemical rays for which Bunsen 

 and Roscoe have devised a method of measurement, the manipulation of which, 

 however, for our purposes would entail great difficulties. Since it results from 

 the determinations made by the observers mentioned, that the " chemical intensity" of 

 daylight, in general rapidly increases from sunrise to mid-day, to decrease thence 

 till sunset again with similar rapidity, and since this suffices in the meantime for the 

 purpose pursued by me, I have not undertaken any photometric measurements.' 



From these considerations, now, we may preliminarily, at least presumably, 

 put together what course the rapidity of growth under the influence of varying 

 moisture, temperature and illumination may possibly assume. On this subject I 

 expressed myself as follows : ' If we now attempt, on the basis of the observations 

 made, to form for ourselves an idea of the course of growth (or of its graphic 

 representation, the curve of growth) of an internode, which is exposed to the 

 varying and different causes of growth, especially in the open, it is at once clear 

 that the curve of growth may assume the most various forms, according as the 

 different causes work in unison or in opposition, and. according as the growing 

 member is in this or that phase of its grand period. In order forthwith to dispose 

 of the often raised question whether growth is stronger or feebler during the 

 night than during the day, and to render its meaning clear, we may attempt an 

 analysis of the combinations of causes of growth and their efifects denoted by the 

 words day and night. As a rule the average temperature during the day is higher 

 than the average temperature during the night ; accordingly growth in the daytime 



