GROWTH 199 



growth, and secure a record through its entire cycle. The recording 

 meteorological instruments should stand in the vicinity. 



Precautions. Aside from a sufficient temperature, good light, and free- 

 dom from injurious gases, the most important condition of growth concerns 

 proper watering. The air must be kept at least moderately humid, by copi- 

 ous sprinkling of the experiment house, and the pot must not be allowed to 

 dry out excessively, which may be prevented by enclosing it in a larger one, 

 with wet sphagnum moss between. It is even advantageous to surround the 

 plant under experiment by a thicket of other plants. Since watering causes 

 a swelling of the soil, and hence a slight lifting of the entire plant, with a 

 consequent error in the growth record, the watering should be done either 

 once a day when the readjustment of the instrument is made, or, better, 

 should be effected by a hydrostatic or self-regulating device (described in 

 Part III), of which Livingston's should be by far the best for this purpose. 

 Or, in some special cases, it may be possible so to enclose the plant in a sat- 

 urated atmosphere that a single thorough watering at the start will suffice 

 to the end of the experiment. 



Auxoscopes, Auxanometers, and Aidcographs. Of these there are 

 many forms which may be classified as follows. Auxoscopes are of subor- 

 dinate importance, and will be considered under Demonstration Methods 

 below. 



Auxanometers. Of these there are several types, applicable to different 

 phases of growth as follows: First, and most exact, is the horizontal micro- 

 scope. It stands on a firm tripod base, can be levelled, is adjustable for 

 height, has an objective of long focus with an ocular micrometer, and is 

 provided either with a vemier scale or a screw micrometer. In use it is 

 kept permanently beside the growing object, whose increase in height is read 

 in part on the ocular scale and in part on the vernier. Owing to the inter- 

 ference of leaves, with their sundry nutations, etc., it is better adapted for 

 use with small leafless structures, such as molds and roots. Its use for 

 growth measurement seems to have been introduced by Sachs in 1872; it 

 was improved by Pfeffer, whose form is obtainable from Albrecht of 

 Tubingen. Wiesner has also a form (Zeitschrift fur Mikroskopie, 1893, 

 147), as has F. Darwin (Darwin and Acton, 153), the latter obtainable 

 from the Cambridge Scientific Instrument Company. A form by W. Wil- 

 son of London permits both vertical and horizontal measurements. An 

 excellent instrument designed by Barnes is sold by the Bausch & Lomb 

 Optical Company; another is offered by Leitz. A Cathetometer, used 

 in Physics, is also admirable for this purpose, as Sachs pointed out. An 

 adapted arrangement, utilizing two ordinary microscopes, has been described 

 by Richards (Torreya, 3, 1903, 136). Second, there is the cup micrometer, 

 invented by F. Darwin (Darwin and Acton, 150), and supplied by the 

 Cambridge Scientific Instrument Company. A needle, attached by a silk 

 thread passing over a pulley to the tip of the plant, makes an easily seen 

 contact with a surface of oil or mercury in a cup adjustable for height by a 

 micrometer screw. But the instrument appears to have no special merits 



