54 STRUCTURE AND PHYSIOLOGY OF SEED PLANTS 



B. Determine the relative proportion of the maximum illumination of 

 sunlight needed to bring about positions 1, 2, and 3. The light meas- 

 urements are to be made by means of ordinary photographic printing 

 paper ("solio" paper answers well) as follows: cut the paper in a 

 very dark room into pieces about an inch square and at once put them 

 into small pasteboard or tin boxes and shut them away in a close drawer 

 or a windowless closet. All the paper in each box must be cut from the 

 same sheet of sensitive paper. One square of paper may be marked with 

 a violet aniline pencil and then exposed, at about noon, to the rays 

 out of doors, so that they will strike it vertically. Note exactly with a 

 watch in how many seconds the pencil mark nearly disappears. The 

 paper should then be at once shut up in the box from which it was 

 taken. This darkened square of paper may now be used as a standard. 

 If it darkened in 30 seconds, and another square used to measure illu- 

 mination (1) darkened to the same tint in 2400 seconds, then illumina- 

 tion (1) was -^ full sunlight, or 1.25 per cent. 



RECORD 



Highest illumination for position 1 



Average illumination for position 2 



Least illumination for position 3 



What is the apparent object of these movements ? What other plants have 

 as many positions as the bean and the locust ? 



Eeference, Pfefler-Ewart, 31, IIL 



EXPERIMENT XXIX 



Can growing leaves adapt their positions to new light relations ? * * 

 Select a young, leafy, vertical branch of maple growing out of 

 doors, or a vigorous young sunflower {Helianthus) plant growing 

 out of doors or under nearly vertical light.' Bend the shoot into 

 a horizontal position and note, whether the leaves adapt them- 

 selves to their new relations. If there is any adaptation, describe 

 exactly the "leaf movements by which it is brought about. 



Eefeeencb. Pfeffer-Ewart, 31, III. 



1 Less satisfactory studies can be made of geraniums, begonias, or other plants 

 grown in the window and turned at intervals of several weeks. 



