SPONTANEOUS MOVEMENTS OF GROWTH 249 



tensions in their tissues may be set up^lay inequalities in the 

 turgidity of, the various component cells as well as by irregular 

 growth : both causes play a part in many instances of plant 

 movements. 



Ex. 147. — (i) On a warm day when there is no wind examine some young 

 plants of scarlet-runner beans, hops and other twining plants which are 

 growing round upright poles or string. Draw a line on the ground from the 

 base of the pole, in the direction in which the tip of the stem is found at the 

 time. Examine the plants at intervals of half an hour and similarly marie 

 the direction in which the tip is curved over at these times ; try and deter- 

 mine how long it takes the tip to make a complete revolution round the pole 

 as a centre. 



(ii) Make similar observations on the nutation of the tip of the stems of 

 nmner bean plants grown in large pots and allowed to wind round sticks 

 stuck in the soil. The plants should be placed out of doors, not in direct 

 sunlight. 



Ex. 148. — Place ^ome soaked broad beans with the micropyle downwards 

 in damp sawdust, and allow them to germinate. When their roots are about 

 an inch long take them up and select one with the straightest root. Pin it 

 through the narrowest diameter of the cotyledons to a slender stick or thin 

 piece of wood and place the latter through a hole in a sheet of cork or card- 

 board. Then place the cardboard with the bean attached over the neck of a 

 wide-mouthed bottle containing a very little water, and arrange that the root 

 is vertical within the bottle. 



Stand the whole in a dark cupboard or cover it with a box to exclude the 

 light. 



Examine the root after 12, 24, 36 and 48 hours and see if it remains 

 vertical or if it nutates in any way. 



Does it nutate more in the plane of the cotyledons than at right angles to 

 this plane ? 



Ex. 149. — Cut pieces two inches long from the full-grown stems of a sun- 

 flower and other plants. Carefully measure and then split them into longi- 

 tudinal strips, so as to include in some the pith only and in others the cortical 

 tissues only. Measure the separate strips and compare their lengths with 

 each other and with the original length of the whole piece. 



Note also the form of the separate pieces. 



Ex. 160. — In July or August and at other times remove a complete ring of 

 bark an inch long from three or four-year-old branches of sycamore, birch, 

 beech and willow. Then try and place the bark in its original position on 

 the shoot : does it fit exactly ? 



