SECT. II 



PHYSIOLOGY 



233 



covered with smoked paper, and kept in rotation by the clock-work {V). If the 

 drum is set so that it rotates on its axis once every hour, the perpendicular 

 distances between the tracings on the drum will indicate the proportional hourly 

 growth. 



The grand periods in the growth of an organ, due to the internal causes, are clearly 

 shown by such self-registering auxanometers by the gradual increase and final de- 

 crease in the perpendicular distances, represent- 

 ing the increment of growth. Stilehl found the 

 daily growth in length of a root of Lupine, ex- 

 pressed in tenths of millimetres, to be: 58, 70, 92, 

 97, 165, 192, 158, 137, 122, 83, 91, 59, 25, 25, 8, 2, 

 o. For the first internode of the stem, growing 

 in the dark, the daily growth observed was : 8, 

 9, 11, 12, 35, 43, 41, 50, 51, 52, 65, 54, 43, 37, 28, 

 18, 6, 2, o. 



The grand periods of growth, that is, the 

 gradual increase from nil to a maximum, and 

 the succeeding decrease to nil again, are, how- 

 ever, not evident throughout the whole of a 

 root ; during the growth in length only a small 

 portion of a root is actually, at one time, in 

 process of elongation. In roots of land-plants 

 the growing region extends over only about one 

 centimetre of the extreme tip, often indeed 

 over only J centimetre ; while all the rest of the 

 root has already completed its growth in length. 

 This may be made clear by marking off with 

 india-ink, near the tip of a root, narrow zones 

 of equal width, which would thus also be made 

 up of cells of nearly equal size. In Fig. 195, I, 

 is shown a germinating Bean, Vicia Faba, whose 

 root - tip has been marked in this way ; Fig. 

 195, II, represents the same root after 22 hours 

 of growth. The marks have become separated 

 by the elongation of the different zones, but in 

 different degrees, according to their position. 

 The greatest elongation is shown by the trans- 

 verse zone 3 ; from there the growth in length 

 decreases towards the younger zones (2 and 1), 

 as well as towards the older (4 to 10). This 

 peculiar distribution of growth is but the result 



of the grand periods of growth of the cells in zones of different ages. In the 

 millimetre-broad zones of a root of Vicia Faba Sachs found, after twenty-four hours, 

 that the increase in growth, expressed in tenth-millimetres, was as follows : — 

 Zones : I., II., III., IV., V., YI., VII., VIII., IX., X., XI. 

 Increase : 15, 58, 82, 35, 16, 13, 5, 3, 2, 1, o. 



The elongating region in shoot axes is generally much longer than in roots, 

 and usually extends over several centimetres, in special cases even over 50 or 

 more centimetres. The distribution of the increase corresponds in stems, as in 

 roots, with the grand periods of growth of the cells. Even by intee'calary growth, 

 where the region of elongation is not confined to the apex but occurs in any part of 



Fig. 195. — Unequal growtli of different 

 regions of the root-tip of Vicia Faba. 

 I, The root-tip divided by marking 

 with india-ink into 10 zones, each 

 1 nun. long. II, The same root after 

 twenty-two hours ; by the unequal 

 growth of the different zones the 

 lines have become separated by 

 unequal distances. (After Sachs.) 



