C INTRODUCTION. 



lower surface, and thus causes it to bend downwards. Hyi>&- 

 nasty is the reverse, and implies increased growth along the 

 lower surface, causing the part to bend upwards.* 



Methods of Observation. The movements, sometimes very 

 small and sometimes considerable in extent, of the various 

 organs observed by us, were traced in the manner which after 

 many trials we found to be best, and which must be described. 

 Plants growing in pots were protected wholly from the light, 

 or had light admitted from above, or on one side as the case 

 might require, and were covered above by a large horizontal 

 sheet of glass, and with another vertical sheet on one side. A 

 glass filament, not thicker than a horsehair, and from a quarter 

 to three-quarters of an inch in length, was affixed to the part to 

 be observed by means of shellac dissolved in alcohol. The 

 solution was allowed to evaporate, until it became so thick that 

 it set hard in two or three seconds, and it never injured the 

 tissues, even the tips of tender radicles, to which it was applied. 

 To tho end of the glass filament an excessively minute bead of 

 black sealing-wax was cemented, below or behind which a bit of 

 card with a black dot was fixed to a stick driven into ths ground. 

 The weight of the filament was so slight that even small leaves 

 were not perceptibly pressed down. Another method of obser- 

 vation, when much magnification of the movement was not 

 required, will presently be described. The bead and the dot 

 on the card were viewed through the horizontal or vertical 

 glass-plate (according to the position of the object), and when 

 one exactly covered the other, a dot was made on the glass-plate 

 with a sharply pointed stick dipped in thick Indian-ink. Other 

 dots were made at short intervals of time and these were after- 

 wards joined by straight lines. The figures thus traced were 

 therefore angular; but if dots had been made every 1 or 

 2 minutes, the lines would have been more curvilinear, as 

 occurred when radicles were allowed to trace their own 

 courses on smoked glass-plates. To make the dots accurately 

 was the sole difficulty, and required some practice. Nor could 

 this be done quite accurately, when the movement was much 

 magnified, such as 30 times and upwards; yet even in this 

 case the general course may be trusted. To test the accuracy 

 of the above method of observation, a filament was fixed to an 



* These terms are used in the ' Wiirzburg Arbeiten,' Heft ii. 

 sense given them by De Vries, 1872, p. 252. 



