150 GERMINATION, GROWTH, TISSUE TENSION. 



shrunk considerably, pull them off). Does the removal of 

 the foliage-leaves check the growth of the plant, as com- 

 pared with that of similar plants left untouched? In 

 which case ( A or J5) is the effect greater ? 



III. SOME EXPERIMENTS ON GROWTH. 



193. Measurement of Rate of Growth. Seedlings 

 of Broad Bean, Pea, and Phaseolus (French Bean or 

 Scarlet Eunner) afford excellent material for experiments 

 of the rate of growth of roots and stems. The Broad Bean 

 and Pea seeds should in most cases be placed with the 

 hilum downwards; the Phaseolus seeds should be laid 

 horizontally so that the root will grow out at right angles 

 to the long axis of the seed. To avoid heliotropic curva- 

 ture, grow the seeds in darkness ; the temperature should 

 be kept as uniform as possible, at about 20 C. 



194. Daily Growth of Root. Place six soaked Peas 

 in a shallow dish of wet sphagnum, or simply with water 

 half covering them ; label each seed with a number or 

 letter on a small piece of paper fixed by a pin through the 

 cotyledons. Keep in darkness, and at the same hour each 

 day measure off and record the length of each root. Note 

 that (1) there are individual differences between the seed- 

 lings in the daily increments in length ; (2) in each case 

 the daily growth of the root is at first slight, then gradually 

 increases until it reaches a maximum (usually by about 

 the eighth day in Peas at 20 C.), and then gradually falls 

 off again. Plot the measurements on squared paper and 

 construct the curve showing the rise and fall in the rate of 

 growth in length. 



195. Grand Period of Growth. A similar result is obtained 

 with all growing organs. The rate of growth of a growing organ 

 (root, stem, leaf, etc.) is not uniform, and the same applies to each 

 of its constituent cells. A growing structure, even under constant 

 external conditions, does not undergo equal amounts of growth in 

 equal successive time intervals. When growth begins, its rate is at 

 first slow ; then it gradually becomes accelerated until a maximum 



