754 PLANT GROWTH lO 



organ, usually a shoot, by a very light thread to a lever of large ratio, which then 

 marked on a drum a magnified record of the growth. The device is called an auxa- 

 nometer. This method does not give localization and is suitable only for relatively 

 large organs. With its use several workers showed that shoots elongate more rapid- 

 ly in the dark than in the light. In recent years more sensitive auxanometers have 

 been devised for precise measurements on seedlings growing under controlled 

 conditions. These depend on the closure of a contact by the growing shoot; this 

 causes the contact to move away a fixed distance and the motion is recorded in 

 some way. An example is the Koningsberger auxanometer, used by Dolk (1936) in 

 studying how the growth rate alters when seedlings are placed horizontal. A more 

 elaborate form was recently described by Idle (1955-56). These instruments can 

 be operated in complete darkness. 



More commonly, straight growth is measured by direct observation through a 

 traveling microscope. The method is precise and the time periods can be short, 

 though again the method does not give localization. Unfortunately, of course, 

 illumination is required, although for many purposes a weak red or green light 

 has little or no influence on the growth. 



Growth can be measured by the increase in fresh weight, especially with isolated 

 pieces of tissue, which can be readily blotted and weighed. This method has been 

 used with tissue cultures ( e.g. by Heller, 1953) and with disks of tuber tissue [e.g. 

 by Reinders, 1942, and many later workers) and seems to have few disadvantages, 

 beyond the difficulty of maintaining sterility during the transfers. It is, however, 

 essential to avoid reversible gain or loss in weight by wetting or drying; in other 

 words, the tissues have to be at their natural turgor. 



Instead of the gain in weight of the tissue, the loss in weight or volume of the 

 medium could be measured, but this has seldom been done. Cholodny (1933) 

 measured the uptake of water by an isolated Avena coleoptile by following the move- 

 ment of a water meniscus in a capillary tube connected to the base of the shoot. 



Curvature often provides a sensitive growth response. A small difference in 

 growth between the two sides of a shoot or root, or a fungus hypha, gives rise to an 

 easily measurable curvature, the faster growing side becoming, of course, the 

 convex side. In the Avena coleoptile a growth increment of 150 \x. on one side 

 appears as a curvature of about 10°. This can conveniently be measured with an 

 error of ^ i . Such curvatvire methods have been widely used. The curvature can 

 be produced either by applying the growth stimulant to one side only, as in the 

 Avena test (Went and Thimann, 1937) and in its counterpart using lanoline 

 (p. 756) or by applying it uniformly and making use of the inherent difference in 

 response of different layers of tissue, as in the slit pea stem test (see below). 



For unicellular algae the methods of bacteriology are applied — the hematocrit 

 tube, turbidity measurements, or various types of counting plate. For fungi, most 

 observers have washed the mycelium and determined its dry weight; this involves 

 the assumption that the dry weight per unit volume is constant, or that assimilation 

 of solutes does not proceed without enlargement. Neither of these assumptions 

 appears to have troubled workers with fungal mycelia, and the error involved 

 may not be very large. Indeed it is partly a matter of definition, since the growth 

 of fungi (though not of green plants) might be defined in terms of gain in dry 



