MOVEMENTS OF GROWTH. 403 



A few potatoes are dug up in August, and immediately put into 

 a thermostat at a temperature of C. (see 49). After about 

 four weeks the potatoes are placed in flower-pots in loose, moist 

 garden earth, and exposed to conditions favourable to germina- 

 tion, light being excluded. The development of the buds at once 

 begins, while control experiments teach that tubers not previously 

 cooled do not by any means germinate in autumn, but much later. 

 We have, therefore, by cooling the potatoes, eliminated their rest- 

 ing period. It has been shown in 126 that at low temperatures 

 the tubers collect in their tissues considerable quantities of sugar, 

 since under these circumstances the respiration of the cells is very 

 feeble. After the cooling, a fairly large quantity of plastic 

 material stands at the disposal of the buds, and they can there- 

 fore rapidly develop. In this connection some observations which 

 I myself made on Pavia twigs are also of interest. Pavia twigs, 

 cut in the middle of January, placed with their lower end in 

 water, and left in the hothouse, unfolded their buds in the middle 

 of March. Twigs of Pavia cut at the end of October, and taken 

 into the hothouse, did not open their buds till after the middle of 

 March. The comparatively rapid development of the buds not 

 placed in the hothouse till January is perhaps due to their being 

 able, while in the open, to accumulate sugar in their tissues, in 

 consequence of the lower temperature to which they were then 

 exposed. Twigs removed to the hothouse, as early as October 

 have also perhaps produced sugar ; but this, if not very abundant, 

 would be used up in respiration, so that the buds would be unable 

 to unfold till March. At this time certainly a specially energetic 

 production of diastatic ferment takes place in the twigs ; the 

 quantity of sugar now formed is sufficient not only for the main- 

 tenance of respiration, but also to enable the buds to begin to 

 develop. 3 



The following observation is also of interest: The willow (Salix 

 fragilis) mentioned on p. 8, which was reared by water culture, 

 and stood all the winter in a warm room, kept perfectly healthy, 

 but did not unfold its buds till March 26, 1895. Also twigs 

 cut from this willow and placed in water have only formed new 

 shoots within the last few days. On the other hand, shoots re- 

 moved in the middle of December from plants growing in the open, 

 when placed in the warm room in water, produced roots and new 

 shoots in the course of four weeks. 



