256 



BOTANY 



PART I 



In order to measure the growth in length of a plant, it is customary to magnify 

 in some way the actual elongation for more convenient observation. This may be 

 effected by means of a microscope, which magnifies the rate of growth corre- 

 spondingly with the distance grown. For large objects, the most convenient and 

 usual method of determining the rate of growth is by means of an AUXANOMKTKU. 

 The principle of all auxanometers, however they may differ in construction, is the 



same, and is based upon the magnification of 

 the rate of growth by means of a lever with a 

 long and short arm. In Fig. 216, at the left, a 

 simple form of auxanometer is shown. The 

 thread fastened to the top of the plant to be 

 observed is passed over the movable pulley (r), 

 and held taut by the weight (</), which should 

 not be so heavy as to exert any strain on the 

 plant. To the pulley there is attached a slender 

 pointer (Z), which is twenty times as long as the 

 radius of the pulley, and this indicates on the 

 scale (S) the rapidity of the growth, magnified 

 twenty-fold. By a growth in the length of the 

 plant-stem of mm., the pointer would accord- 

 ingly register 4 mm. 



Self-registering auxanometers are also used, 

 especially in making extended observations. In 

 Fig. 216, at the right, is shown one of simple 

 construction. The radius of the wheel () corre- 

 sponds to the long arm, and the radius of the 

 small wheel (r) to the short arm of the lever, in 

 the preceding apparatus. Any movement of the 

 wheel, induced by the elongation of the shoot, 

 and the consequent descent of the weight (G), is 

 recorded on the revolving drum ((7) by the pointer 

 attached to the weight (Z), which is, in turn, 

 balanced by the counterweight ( W}. The drum 

 Fio. 217. Unequal growth of different j s covered with smoked paper, and kept in rota- 

 regtons of the root-tip of Vicia Fa> M . [m , the clock _ work {f7)> If the drum fe 8et 

 /, The root-tip divided by marking . . , 



with india-ink into 10 zones. e;u-h *> that it rotates on its axis <>nct> every hour, 

 l mm. long. 17, The same root after the perpendicular distances between the tracings 

 twenty-two hours ; by the unequal on the drum will indicate the proportional hourly 

 growth of the different zones the grow ^i, 



lines have become separated by mi- , . 



equal distances. (After ACHS.) 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 decrease in 

 the perpendicular distances representing the increment of growth. STREHL found 

 the daily growth in length of a root of Lupine, expressed 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 *aa : 

 8, 9, II, 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 zero to a maximum, 

 and the succeeding decrease to zero again, are, however, 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 



