ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 631 



quence of continuous illumination, be suddenly darkened and then 

 kept in darkness, the leaves first perform the sleep-motion, and after 

 a time expand again. The first motion is the simple stimulus motion, 

 or, as Pfeffer calls it, " reception motion," which in periodically- 

 moved leaves, is always produced by sudden changes of illumination. 

 The plant, however, does not remain stationary, but the movements 

 are several times repeated, in about equal daily periods, but with 

 even less energy. In this way Pfeffer obtained, by simple darkening 

 of the previously quite motionless leaves, curves of motion which are 

 very similar to those given by Baranetzky for the growth of Gesneria. 

 Pfeffer, therefore, well compares the periodically moved leaves to a 

 pendulum, which, after receiving an impulse, makes a whole series of 

 rhythmic oscillations. 



That the periodicity of the growth in length is an altogether 

 similar phenomenon, Baranetzky proved by an experiment, in which 

 he exposed to light for 10 hours a Gesneria which had remained in 

 darkness till every variation had disappeared, and then brought it into 

 darkness again ; a daily period could then only be indistinctly per- 

 ceived by the points of the curve. The plant was then exposed 

 16 hours to the light, and again brought into darkness; and now the 

 periodicity appeared quite regular, with all its characteristic pecu- 

 liarities. The 10 hours' illumination was thus not sufficient to call 

 forth the extinguished periodicity, but an illumination of 16 hours was 

 fully so ; the following darkness called forth a whole series of varia- 

 tions, each of which took about 8 to 10 hours, 



Herr Kraus has also recently investigated the daily variations in 

 the dimensions of thickness of tree-stems. His measurements were 

 made chiefly on dicotyledonous trees, but also on Draccena Draco and 

 Pinus Strohus. * The results are briefly as follows : — (1) The stems 

 of trees are subject to a daily recurrent, regular variation of their 

 diameter. (2) The diameter diminishes steadily from the earliest 

 morning hours to the early afternoon, at that time reaching a mini- 

 mum. There is then a constant increase of diameter to a first (small) 

 maximum when darkness comes on. After sinking a little, the 

 dimension increases again till dawn, when a (great) maximum is 

 reached. (3) This course of variation thus coincides exactly with the 

 course of variation in tension. (4) The curves show that the progress 

 of variation of diameter, and that of temperature, are, on the whole, 

 inversely correspondent ; to rise of temperature corresponds a diminu- 

 tion of the diameter, and conversely. (5) The maxima and minima, 

 however, do not quite coincide ; the nightly maximum thickness 

 occurs long before the minimum of temj)erature of the night, and the 

 day-maximum of temperature before the minimum of the thickness. 

 Then, besides temperature, other factors must operate. 



In stems of 40-50 mm. the differences in size were mostly under 

 0*5 mm. 



Freezing of Plants-f— H. Miiller-Thurgau has applied low 

 temperatures to plants, with the view of ascertaining the effect of 



♦ Bot. Zt^., xxxviii. (ISSO). 



t NaUu-forscher, xiii. (18S0) pp. 121-3. 



