288 ScHREINER AND REED: EXCRETIONS BY ROOTS 
PRELIMINARY EXPERIMENTS 
The first experiments were designed to perfect a method of using 
the plant so that it would serve as an indicator of the deleterious 
root excretions, if such substances were produced. 
Several sorts of perforated tubes of glass and mica 
were tried; in the end it was found that more satisfactory 
results were obtained from the use of glass tubes having 
an internal diameter not greater than I cm. The best 
results were obtained from the use of segmented glass 
tubes, having small openings between the segments 
(FIGURE I). The tubes were made from glass tubing 
having an internal diameter of 6 or 8 mm. Pieces of 
tubing 10 cm. long were closed at one end by fusion. 
Each of these pieces was then cut into three nearly equal 
lengths. These three segments were then bound to a 
adjoining segments. They thus formed a straight seg- 
mented tube with narrow openings about one-third of 
the distance from the top and bottom respectively. 
The segmented tubes were placed in a vertical posi- 
tion in small glass jars, the fused ends of the tubes rest- 
ing on the bottom of the jar. Pure non-nutrient agar, 
which had been washed in three changes of distilled 
water and melted in distilled water, was poured into the 
jars until its level reached the tops of the tubes. When the agat 
had cooled to a temperature between 35° and 38° C., the roots 
of a wheat seedling three days old were inserted in the open, 
upper end of each segmented tube. ’ 
The design of such an arrangement was to enclose the grow- 
ing roots in a small space in which the toxic excretions would be 
confined. At the same time, it provided at intervals small open- 
ings through which there would be some diffusion of the toxic 
products to the exterior. When the growing roots reached one 
of these regions from which diffusion was taking place, they would 
have a chance to respond to this unequal distribution of the dele-_ 
terious substances. Such response would be plainly manifested 
by the growth of the roots toward regions where deleterious sub- 
stances were less abundant. In other words, the roots might 
glass rod, leaving a space of two or three mm. between | 
