20 4 PLANT PHYSIOLOGY 



A special form of adapted auxograph for recording growth in length of 

 roots is described by STONE in Botanical Gazette, 22, 1896, 258. 



All, practically, of the existent auxographs require the use of threads 

 for connecting plant and instrument; and in this lies their greatest source 

 of error. For no thread has yet been found which does not alter its length 

 with humidity changes of the atmosphere. In my laboratory I have had 

 repeated efforts made to find such a thread. Waterproofing, either by coat- 

 ing, soaking, or boiling the thread with various waxes, liquid rubber, and 

 oils, does not succeed, or only at the expense of making the threads too 

 stiff to work properly over the wheels. Substitutes of wire, fine chains, and 

 quartz fibers are all impracticable for one reason or another. Threads 

 combined from material of opposite humidity relations suggested by PFEF- 

 FER do not work, for the reason that the materials have no stiffness and 

 hence the shortening strands carry the lengthening strand passively with 

 them. So marked is the hygroscopic shortening of some threads, that an 

 apparent marked fall in the rate of growth registered by an auxograph may 

 be due in reality to shortening of the thread in the drying air, and some- 

 times this error may become so great as to actually make it appear that the 

 plant is shortening. Upon the whole the most practicable thread is of fine 

 twisted (not untwisted) silk, thoroughly waxed with beeswax well rubbed 

 in, and kept just as short as possible; this applies especially to the one con- 

 necting with the plant, for any alteration of this one is of course magnified 

 on the record. Then the plant should also be given as uniform humidity 

 conditions as possible, including, when practicable, a saturated atmosphere. 

 All of the above-described instruments were designed for measurement 

 of growth in length, though some of them are, incidentally, equally applicable 

 to growth in thickness. Of special arrangements for the latter, the more 

 important are these. Of non-recording forms, the best are undoubtedly 

 magnifying calipers, as used by JOST (Berichte der deutschen botanischen 

 Gesellschaft, 10, 1892, 600); some form of micrometer (the ZEISS micrometer) 

 was used by EWART. (Annales du Jardin Botanique de Buitenzorg, 15, 

 1898, 1 88). A modification of the latter was used by F. DARWIN, where 

 he made the micrometer carry a point into contact with a dish of oil placed 

 upon the thickening part (Annals of Botany, 4, 1889, 118, and 7, 1893, 

 468). Of recording forms, MACMILLAN used a movable wooden jacket in 

 connection with the BARANETZKY auxograph (Botanical Gazette, 16, 1891, 

 149, and American Naturalist, 25, 1891, 462), and KATHERINE GOLDEN 

 has described an arrangement whereby a magnifying glass rod, recording 

 on a revolving drum, is pushed laterally by a growing stem held in a Y- 

 shaped support (Botanical Gazette, 19, 1894, 113). 



DEMONSTRATION METHODS. For general student use I have found nothing 

 so effective as the demonstration auxograph above described, recording the 

 growth in length of a flower-stalk of Grape Hyacinth. For demonstration 

 before an audience, and to show growth actually in progress, a good auxo- 

 scope could no doubt be made from the arc-pointer of SACHS, with a very 

 long arm, or, and better, from a mirror arranged to reflect a long beam of 

 light when moved by the growth of the plant. Auxoscopic arrangements 



