68 STUDIES OF THE LEAF 



considerably dried, record the result and the conclusion as to which 

 surface exhales vapor more freely. 



Experiment 17. A growing plant of Nasturtium, which has been 

 standing for several hours in one position so that the light lias steadily 

 come from one direction, is to be observed. Do all the leaves face in 

 one direction ? Or several leaves ? If so, mark the side of the pot 

 toward which they incline with some distinctive mark (e.g., A.B. 9.80). 

 Young leaves, or at least those not declining in vigor, should be chosen 

 for record. In the notebook record the position of one of these leaves 

 diagrammatically, as seen from above. The diagram will consist of a 

 circle, for the pot; a radial line (marked le), for the petiole of the 

 selected leaf; a line across the end of this, for the blade; and an 

 arrow (marked li) outside the circle, for the direction of the principal 

 body of light. 



Note the attitude of the stem, as seen from the marked side of the 

 pot. Represent it by a diagram: make a straight level line for 

 the rim of the pot; another rising from this, for the stem. Record 

 the time. Now expose the plant to strong light from a new direction. 

 Indicate this on the first diagram by a second arrow (li'}- 



Leave till a change is plain. At length indicate the position of the 

 selected leaf by new lines (le') on diagram 1, and the attitude of the 

 stem, as seen from the original side of observation, by a dotted line on 

 diagram 2. If any movements of leaf blades are discovered, find how 

 far they are due to the curvings of the petioles. 



Experiment 18. So-called sleep movements. 



Note the position of the leaflets on seedlings of the Sensitive Plant 

 (Mimosa pudica) when standing in the light. Now place over the pot 

 carefully, without jarring the plants, a box or blackened bell jar, so as 

 to exclude all light. In fifteen minutes or so, uncover carefully. 

 What change in the position of the leaves? Oxalis may be used for 

 this experiment. If Lupine or Bean is used, the time will be longer. 

 They may be left in a dark closet over night. 



Experiment 19. Sensitiveness of Mimosa. 



Use the seedlings of the last experiment. Touch one of the leaflets 

 very gently. Touch others less gently. Note the several effects in any 

 one leaf, and if they occur, the resulting effects on surrounding leaves. 

 Are the cotyledons sensitive? Select a plant which is still in the 

 normally expanded condition. Press a hot needle against one of the 

 cotyledons, without shaking the plant. Wait for the effect. 



If a large plant is available, apply a match flame to the tip of one of 

 the leaves. Note what parts are affected in succession, and the manner 

 in which the effect travels over the plant. Measure the greatest distance 

 to which the effect is transmitted, and the time taken in transmission. 



This experiment may be done before the whole laboratory division, 

 one plant serving for all. If time and facilities permit, it will be of 



