ss^ 



LECTURE XXXII. 



perceived in half-hourly or hourly observations indicate the existence of other internal 

 causes which, likewise independently of external influences, aid in determining the 

 rapidity of growth. I have designated this phenomenon as " discontinuous variations 

 of growth," 



' I have no doubt that the knowledge of the grand period, as well as that of the 

 discontinuous variations of growth, will at some subsequent date be of considerable 

 use for a theory of the mechanics of growth. Here however I have only brought 

 forward both phenomena because a knowledge of them is absolutely necessary to the 

 study of the effects of external influences on growth in length, and because the 

 experimental establishment of causal relations is rendered extremely difficult by 

 them. Given for example the case where a growing internode is observed in con- 

 stant moisture and darkness but with variable temperature, then the differences of 

 growth obtained in considerable periods (several days for example) are not to be 

 taken forthwith as functions of the different temperatures, since the phase of 

 the grand period is altering at the same time ; it may happen that the higher 

 temperature (beneath the optimum) coincides with a smaller hourly or daily growth, 

 because the internode at this time is in a condition where it is less capable of growth 

 generally. It is easy to suppose, in order to avoid this difficulty, the plant ex- 

 posed to different temperatures quickly one after another, and the phase-difference 

 of the grand period thus reduced to a minimum ; but the discontinuous variations of 

 growth which make their appearance quite irregularly might sometimes increase, some- 

 times diminish, the effect of the temperature on the growth, without our being in a 

 position to decide how much is to be placed to the account of one or of the other. Very 

 much the same difficulties would repeat themselves when the temperature is constant, 

 with regard to the action of variable illumination or moisture in short periods of time. 



' This complication with internal disturbances, in cases where the object is to 

 become acquainted with the effects of external agents on growth, makes it not only 

 necessary to multiply the number of observations enormously, but it also entails 

 our being but seldom in a position to derive from the numbers expressing hourly 

 growth any causal connection at all ; in order to ascertain this it is necessary to draw 

 these numerical values as co-ordinates, and the curves, properly constructed, then 

 generally reveal the causal relations.' I then showed further in what manner the 

 moisture of the environment may co-operate in influencing the rapidity of growth, by 

 means of its influence on the turgescence of the cells, and how in considerations of 

 this kind this fact has to be borne in mind. In the second place the influence of 

 temperature on growth required quite special and closer consideration, which I gave 

 in the following words : — ' That growth only begins when a certain lower temperature 

 (the specific zero point) is passed, that it is accelerated so much the more the higher 

 the temperature is, and that at a certain higher temperature, the optimum temperature 

 (between 20° and 30° C), a maximum rapidity of growth occurs, and that, moreover, 

 as the temperature rises still further the growths again decrease, I have already 

 shown to be true for seedlings, and Koppen has confirmed it in his work quoted. 

 Moreover Harting (1842) had already inferred from his observations a similar 

 behaviour on the part of the shoots of the Hop, but without contributing convincing 

 proofs of it. 



' These facts are to be realised, so far as the problem lying before us is 



