SPONTANEOUS MOVEMENTS OF GROWTH 247 



direct rays of the sun, and the other set near them but covered with boxes 

 which exchide all light. 



(i) From time to time measure and compare the diameters of the stems and 

 the lengths of the internodes of the plants growing in the light with those Oi 

 the plants growing in the dark. 



(ii) Measure and compare the length and breadth of the leaves of the two 

 sets of plants. 



(iii) Note the differences in the colour and firmness of the two sets of plants. 



Ex. 146. — Make observations similar to the above on the shoots developed 

 in light and in darkness respectively, from the tubers of the potato and 

 artichoke, those springing from the roots of the dahlia, and the leaves 

 of onions. 



3. Spontaneous movements of growth : nutation and tissue- 

 tension. — Growth rarely proceeds evenly in all parts of a shoot, 

 root, or other organ of a plant ; certain portions grow more 

 rapidly or continue to grow for a longer period than adjoining 

 parts. In consequence of this uneven growth the organs of 

 plants (i) exhibit peculiar, slow, spontaneous movements, and 

 (2) their tissues become subjected to pressures and tensions in 

 various directions. 



In stems and roots the growth of one side is more rapid than 

 the other : the more rapidly growing side becomes slightly longer 

 than the other, and the whole growing part forming the end of 

 the stem or root becomes bent or curved in consequence. 



The side on which most rapid growth occurs is not always 

 the same but varies from hour to hour, so that the growing 

 organ bends over in different directions and the tip travels 

 slowly round and round, following a spiral line in its upward 

 or downward growth. Movements of this kind are spontaneous 

 and automatic : like the rise and fall in the rate of growth 

 during the grand period they originate within the growing 

 organ itself and occur whether the plant is kept in darkness or 

 exposed to the light. 



To such slow nodding movements the term nutation is 

 applied. 



In most stems and roots their tips travel round from light 



