DIRECTION OF GROWTH 221 



Effect of ligM. — We might expect that a stem exposed to 

 the light would grow more rapidly than one kept in darkness. 

 But our anticipations are not realised. Light retards the 

 growth in length of a stem. In the case of the majority of 

 plants which possess stems having internodes of easily per- 

 ceptible length, the stem grows more rapidly in darkness. 

 The plants grown in absence of light have thin stems with 

 long internodes and small leaves ; furthermore, they are devoid 

 of chlorophyll. Such plants are said to be etiolated. In 

 the case of plants like the Iris or Onion, with more or less 

 erect radical leaves, the stem does not elongate abnormally 

 in darkness, nor are the leaves dwarfed. 



Nutation. — Even if every external influence (light, tempera- 

 ture, etc.) be kept unchanged, a stem does not grow evenly at 

 its tip. Just behind the apex it grows more rapidly on one 

 side than on the other, so that at this region the stem becomes 

 a little longer on the more rapidly-growing side, and therefore 

 bends over towards the opposite side. Soon the elongating 

 part of the stem proceeds to grow more rapidly on another 

 side, and the bend takes a new direction. Thus the end of the 

 stem may be said to nod slowly, and the word nutation (nod- 

 ding) is used to denote this phenomenon. Each growing part 

 of the stem, which was originally bent by nutation, finally 

 straightens itself before ceasing to elongate, so that a mature 

 stem does not show the zig-zag course which it executed. This 

 nutation of the stem may be seen especially clearly in twining 

 plants. 



All these variations in the rate of growth in length may be 

 followed by using the method of making equidistant ink marks 

 along the elongating parts of the stems (see page 5). 



CIRCUMSTANCES INFLUENCING DIRECTION OF THE 

 GROWTH IN LENGTH OF STEMS AND ROOTS. 



Various external agencies influence not only the rate at 

 which stems and roots elongate, but, to a certain extent, 

 determine the direction in which those organs shall grow. 



Heliotropism. — If potted plants be left for days undisturbed 

 in front of a window, we know that their stems tend to point 

 towards the window — that is, towards the light. Ordinary stems 

 of flowering plants tend to grow towards the light, and _ to 

 place themselves in a straight line with the source of the light. 



