154 BOTANY FOR BEGINNERS CHAP. 



EXPT. 144. Fill two plant pots with soil, and sow a few Mustard 

 seeds in each. Keep the soil moist. Place one pot in a window and 

 the other in a dark cupboard. Measure the length of the plants in each 

 pot from time to time. Note 



(i) The plants kept in the dark cupboard are yellow in colour ; those 

 exposed to light are green. 



(ii) Those grown in the dark increase in length nearly three times as 

 fast as those grown in the light. 



(iii) The leaves of the plants grown in the dark are very small, but 

 those produced in the light are far larger. 



(iv) The plants kept in the dark begin to droop and soon die ; those 

 grown in the light are healthy and strong. 



(v) The plants in the dark are often attacked by fungi. 



(vi) Light is necessary for the healthy growth of plants, but they 

 grow faster in the dark. 



Growth in Length of Plants. At the apex of a shoot or 

 root are two zones of growth. At the extreme apex of the stem 

 there is a meristematic layer where new cells are produced by 

 division. Just behind this region the cells 'increase in size, but 

 little cell division takes place. 



Most herbaceous plants from time to time show a change in 

 the vigour of the growing point. If a plant like the Deadnettle 

 is examined, the nodes at the base are seen to be crowded to- 

 gether ; that is, the internodes are short. Higher up the inter- 

 nodes are longer, while again towards the apex the nodes are 

 crowded together and the internodes are short. The variation 

 in the length of the internodes depends upon the strength of the 

 growing point. At first the growing point is not very vigorous 

 and it produces short internodes ; as it gains strength, longer 

 and longer internodes are produced. Later, its strength or 

 activity again declines, and the internodes become shorter until 

 the period of growth has ceased. Similarly in the life of most 

 plants there is a grand period of growth. 



Monocotyledonous plants, like the Indian Corn, show this 

 increase in the vigour of the growing point to perfection. If the 

 plumule of a germinating seed of Indian Corn be examined it 

 will be found to be about J of an inch in diameter. If the dia- 

 meter of a mature stem be measured it will be found to be many 

 times larger. How has this increase in size been produced ? 

 The growing point at first could only form a thin stem, but as 

 its strength increased, a larger and larger stem was produced. 

 But, here again, the vigour of the growing point declines later in 

 life and the stem produced has a smaller diameter. 



