384 PHYSIOLOGY OF GROWTH. 



the roots now and then with a little water, or line the cylinder 

 with moist blotting-paper. We place the cylinder in the dark, 

 and expose the seedlings to a temperature as constant as possible 

 (e.g. 20 C.). We may also allow the roots in the cylinders to grow 

 in water. We use vessels of about 3 litres capacity, half filled 

 with water, fix the seedlings on very long pins, and let the roots, 

 but not the receptacles of reserve material, dip into the water. 



After twelve or twenty-four hours it can readily be determined 

 by measuring that the growth in the youngest partial zone, next to 

 the growing point, has not been excessive. In thejiext zone more 

 energetic growth is exhibited. It is one of the following zones in 

 which most rapid growth has proceeded. Then come zones in 

 which it is found that the growth has been slower again, and the 

 oldest parts of the roots have undergone no growth at all (see Fig. 

 127). The older partial zones have already passed beyond the 

 stage of strongest growth ; the youngest 

 have not entered upon it. Each transverse 

 zone of a structure, like that organ as a 

 whole, at first grows slowly, then more 

 rapidly, attains a maximum rate of growth, 

 and finally grows more slowly again. 

 Hence we find also that the youngest trans- 

 -erse zones o a root, which e.g. during the 

 lines have been painted. On first twelve hours have not grown very 

 S^te^^r g" """Oh, daring the following twelve hoars 

 ingfor sometime. already grow more actively. At a particular 



time naturally the maximum of growth lies in these zones, but 

 later still the rapidity of growth diminishes again. It is instruct- 

 ive to observe the growth of the partial zones of a root for a 

 considerable period, determining the increments of growth at 

 intervals (which must, however, be comparatively short, say six 

 to ten hours). A difficulty, which, however, is not insuperable, 

 lies in the fact that the ink-lines placed on the root become 

 disintegrated.* 



In investigating the growth of the root, the student will not 

 escape the important fact that the length of the growing region, 

 i.e. the length of that portion of the originally marked region 



* See Sachs, Arleiten d. botan. Inst. in Wurzburc/, Bd. 1, p. 121. To avoid 

 mistakes it is better each time to make a fine ink-line in the middle of the 

 marks, and use this line as the point of reference for the further measurements. 

 Consult Sachs also for further particulars respecting the methods here indicated. 



