MOVEMENTS OF GROWTH. 411 



seedlings of Phaseolns in flower-pots in darkness. When the 

 primordial leaves have developed to a certain extent, we select two 

 plants, a and 6, whose leaves are as nearly as possible of the same 

 size, and measure their length and breadth, a is still left in dark- 

 ness ; b is exposed for about two hours each day during eight 

 days to weak diffused light, but otherwise also remains in darkness. 

 The leaves of 6, being only illuminated each day for so short a 

 time, do not become green. The leaves of a remain small, while 

 those of b grow considerably.* 



1 See Godlewski, Botan. Zeitung, 1879. 



2 See H. de Vries, Botan. Zeitung, 1879, p. 852. 



3 See G. Kraus in Pringsheim's Jahrbiicher, Bd. 8. Increased multiplication 

 of the cells, as well as increased elongation of the cells, plays a part in the 

 production of etiolation. 



4 For the literature on etiolation, see Detmer, Vergleichende Physioloyie des 

 Keimungsprocesses d. Samen, 1880. 



166. The Influence of Light on Growth. 



It is well known that light exerts a retarding influence on the 

 growth of the most various plant structures. To prove this fact 

 we make the following observations. A considerable number of 

 well-developed pea seeds are soaked and germinated in a box filled 

 with damp sawdust. When the main roots of the seedlings have 

 attained a length of 2 cm. we remove them from the sawdust, and 

 mark them, in the usual manner, at a distance of 10 mm. from the 

 root tip, with fine ink-lines. Special care must be taken that only 

 very similar and perfectly normal seedlings are employed for 

 further investigation. The culture of the plants is continued in 

 glass cylinders, about 25 cm. deep and 10 cm. in diameter, which 

 are provided with suitable wooden covers pierced with a number 

 of holes, and which are filled with spring water. We take two 

 such cylinders. Each is fitted up with a fair number of the seed- 

 lings (perhaps ten to fifteen), which we fix in the holes of the cover 

 by means of cotton wool, and in such a way that their roots dip 

 into the water. The roots in one of the culture cylinders are left 

 exposed to the light; it is desirable to place a mirror close behind 

 the cylinder and parallel with the window, or to rotate the cylinder 

 slowly on a clinostat, so as to prevent any heliotropic curvature of 

 the roots. The other cylinder is covered with black paper, so 

 that the light cannot reach the roots. We conduct the observa- 

 tions in summer, in a room with a north aspect, and at as high a 



