456 The Physiology of Plants Book hi 



by Velten, in 1876, to show similar minimum, optimum, and 

 maximum points, the optimum and maximum being very 

 near together, but differing slightly in the case of different 

 plants. 



Strasburger, two years later, showed that the zoospores 

 of Haematococcus are motile at temperatures as low as 

 o° C. ; their optimum point is from 30°-40° C. and at 50 

 they are arrested. In the case of marine algae the maxi- 

 mum point is about io° C. lower. Kjellman, in 1875, 

 determined the lower limit for the latter forms to be -i-8° C. 



The nyctitropic movements of Oxalis were found by 

 Pfeffer to be only slightly influenced by a fall of as much 

 as io° C, and the resulting disturbance largely depended 

 on the point of their daily period at which the stimulation 

 was applied. Millardet, in 1869, observed a closer relation- 

 ship between temperature and movement in the case of 

 Mimosa petioles, which he found to rise with an increase 

 and to sink with a fall of temperature. 



The influence of the external conditions was proved 

 to have a powerful effect in some cases in determining 

 certain features of the constitution of particular plants. 

 Zimmermann showed in 1882 that the dorsiventrality of 

 the thallus of Marchantia is due to light. Less conspicuous 

 changes were shown by Frank in 1870, when he ascertained 

 that shoots of various coniferous trees, normally radially 

 organized, become dorsi ventral under the same influence. 

 Two years later De Vries observed that in whatever position 

 a lateral branch of one of these was allowed to develop 

 from the bud, the upper surface became the dorsal and 

 the lower one the ventral. Gravitation seemed to play a 

 part in the matter, as the same change happened in dark- 

 ness. 



We have seen that Pfeffer showed that epinasty and 

 hyponasty can be induced by variation of temperature, as 

 in the opening and closing of flowers. Detmer, in 1882, 



