INSTRUMENTS 25 
WATER-CONTENT 
40. Value of different instruments. The paramount importance of water- 
content as a direct factor in the modification of plant form and distribution 
gives a fundamental value to the methods used for its determination. Au- 
tomatic instruments for ascertaining the water in the soil are costly, in ad- 
dition to being complicated, and often inaccurate. For these reasons, much 
attention has been given to developing the simpler but more reliable methods 
in which a soil-borer or geotome is used. The latter is simple, inexpensive, 
and accurate. It can be carried easily upon daily trips or upon longer re- 
connaissances, and _ is 
always ready for instant 
use. In the determination 
of physiological water-con- 
tent, it is practically indis- 
pensable.. Indeed, the 
readiness with which geo- 
tome determinations of 
water-content can be made 
should hasten the universal 
recognition of the fact that 
it is the available, and not 
the total amount of water 
in the soil, which deter- 
mines the effect upon the 
plant. 
Geotome Methods 
41. The geotome. In its 
simplest form, the geotome 
is merely a stout iron tube 
with a sharp cutting edge ae fy 
at one end and a firmly Fig. 1. Geotomes and soil can. 
attached handle at the 
other. The length is variable and is primarily determined by the location 
of the active root surface of the plant. In xerophytic habitats, generally a 
longer tube is necessary than in mesophytic ones. The bore is largely 
determined by the character of the soil; for example, a larger one is neces- 
sary for gravel than for loam. Tubes of small bore also tend to pack the 
soil below them, and to give a correspondingly incomplete core. The best 
results have been obtained with geotomes of 14-1 inch tube. Each geotome 
