156 BOTANY FOR BEGINNERS CHAP. 



THE IRRITABILITY OF GROWING ORGANS. 



The principal stimuli which act on the growing organs and 

 produce movements are light, gravitation and water. The 

 agencies which act on protoplasm, and the movements which 

 they produce, will here be considered. . 



The Action of Light on the direction of Growth. 

 The importance of light to plant life cannot be overestimated. 

 We have seen how necessary it is for assimilation, and for the 

 healthy growth of a plant. The various parts of plants react in 

 different ways when exposed to light. The aerial portions 

 generally turn towards the light, while those parts which under 

 normal conditions develop in the dark, turn from it. In the 

 case of the former light is necessary for their full development, 

 but the latter can grow without light. 



Heliotropism. The action of light is well shown by win- 

 dow plants. The stems of such plants are not erect as in the 

 open air, but are inclined towards the source of light. This 

 turning of a portion of a plant either towards or away from the 

 light is called heliotropism. 



Positive Heliotropism. The portions of a plant which 

 turn towards the light are said to be positively heliotropic. 

 The stems and leaf-stalks incline towards the source of illumin- 

 ation so as to place their long axis parallel with the rays of 

 light. But the leaf-blades arrange themselves at right angles to 

 the illumination and so receive the maximum amount of light. 



Negative Heliotropism. Those parts of the plant which 

 turn from the light are said to be negatively heliotropic. 

 Roots, rhizomes, and bulbs turn from the light and are conse- 

 quently negatively heliotropic. Aerial roots like those of the 

 Ivy also turn from the light. 



EXPT. 148. Place a pot containing a Castor Oil plant on a window 

 sill, and observe it from day to day. Note 



(i) The stem and leaf-stalks bend towards the sun ; the divided leaves 

 arrange themselves at right angles to the window. 



(ii) If the pot is turned, the. leaf-stalks and leaf-blades move round 

 until they occupy their old position. 



EXPT. 149. Examine a piece of Ivy which is clinging to the wall or 

 to the trunk of a tree. Note 



(i) Most of the clinging roots are developed on theshady side of the stem. 

 (ii) The roots developed in the light are turned away from it. 



