PERIODICITY IN DEVELOPMENT. I 343 



before sunset ; the increase continued until sunrise when the maximum was again 

 reached. This growth curve is easily explained if we assume the increasingly 

 retarding effect of light in the course of the day, and the effect of the withdrawal 

 of light at night. This hypothesis has, however, no experimental foundation 

 and the results obtained by Sachs have no general application. Godlewski, 

 experimenting with epicotyls of Phaseolus, obtained entirely different results ; 

 he found that growth was greater by day than by night and that the maximum 

 was reached between six and eight p.m., the minimum in early morning. He 

 further showed that the transition from darkness to light acts as a stimulus, in 

 consequence of which a sudden but temporary decrease in growth took place. 

 A distinction must at least be drawn between this influence of change in light and 

 the influence of constant illumination or darkness. Corresponding stimulatory 

 activities due to changes in temperature appear to be generally non-existent 

 (True, 1895), but we shall see in Lecture XXXIX that they do occur in special 

 cases. 



Just as we are unable at present to understand fully the alterations in 

 growth taking place under the influence of a simple light change, so we have even 

 greater difficulty in explaining the after-effect of daily periodicity which was 

 demonstrated by Sachs and Baranetzky (1879) as taking place in darkness 

 under constant temperature. These investigators observed in certain cases that 

 variations in growth, exhibited during light variations, continued with the same 

 periodicity in darkness all day long, and there can be no doubt that a causal 

 connexion existed between them. Pfeffer (1881) has advanced the following 

 explanation of these after-effects. It is based on certain phenomena which we 

 shall have to study later (nyctitropism, Lect. XXXIX), and assumes that after 

 a single illumination not only does a simple retardation of growth take place 

 but that this retardation is necessarily followed after a time by an acceleration ; 

 darkness operates in the same way. If, therefore, darkening sets in at a time 

 when, owing to illumination, growth acceleration has already taken place, the 

 effect of the single stimulus is added to the after-effect, and if the total effect is 

 maintained all day long, the after-effects will become all the more established. 

 Evidence is not forthcoming, however, to show that a double alteration in rate 

 of growth takes place at every application of the stimulus, and, further, where it 

 has been observed the second alteration does not make itself apparent till about 

 twelve hours after. This must be the case, however, if, in nature, the new 

 stimulus is to be added to the after-effect of the old. 



Under these conditions it is important to note that periodic movements, 

 especially those with daily rhythm, occur in plants which grow under quite 

 constant conditions, in which an after-effect of any kind is out of the question . 

 Thus Baranetzky has observed a daily periodicity in beets grown in the dark 

 under a constant temperature, resulting from internal factors only, and therefore 

 only accidentally showing a twelve-hourly periodicity. Godlewski also demon- 

 strated in the case of beans germinated in the dark a regular daily periodicity in 

 growth, but this does not always take place, and is entirely absent in the case of 

 certain kinds of seeds. We thus arrive at the conclusion that a daily periodicity in 

 longitudinal growth cannot be due to external factors and their after-effects only. 



Among the phenomena of yearly periodicity the resting period, seen in trees 

 and shrubs, and briefly referred to above, is of special interest. Selecting ex- 

 amples from among our native plants, we readily notice that the quiescent period 

 occurs as a rule in the winter months, the active period in the summer, and one 

 would naturally attribute this to the direct influence of external conditions, and 

 more especially to the annual rise and fall of temperature. Closer examination 

 shows, however, that this view cannot be correct, or at least that the relation 

 between the plant and the environment is not so simple as it appears. 



The winter buds of many trees exhibit the rudiments of an entire shoot. 



