ON THE MYCORHIZAS OF FOREST TREES 
59 
4. The Element of Chance in Mycorhiza Formation 
That the transformation of a root into a mycorhiza depends, 
to a large extent, on chance conditions is shown by the following ob- 
servations by the glass plate method. Six plates were placed on ecto- 
trophic species of trees, and numbered i to 6. Numbers i and 2 
were on Tilia americana; numbers 3 and 4 were on Betula alba var. 
papyrifera; and numbers 5 and 6 were on Carya ovata. They were all 
placed during the month of May, 1912, and the roots under each were 
diagrammed on paper. Plates no. 4 and no. 5 gave no results, because 
air spaces were left beneath them and the roots dried up. Of the 
mycorhizas produced under the successful plates the longest was four 
millimeters and the shortest two millimeters. The lengths of the 
normal roots produced are given below. There was no indication 
in any case of the formation of a mycorhizal cluster, all mycorhizas 
produced being simple and unbranched. The results were as 
follows : 
Plate No. I, on Tilia. — Nine new rootlets were produced betw^een 
June I and July 15. All of these had practically the same appearance, 
and there was no reason for supposing that their fates would be dif- 
ferent. Four of them were dead before the end of July. The other 
five remained alive throughout the season. By the first of October 
two of these had developed normally and had attained lengths of 
4.2 and 7.1 cm. respectively, while the other three had developed into 
typical mycorhizas. 
Plate No. 2, on Tilia. — Four new rootlets were produced. Two 
of them became mycorhizas, and two of them developed as normal 
roots with lengths of 3.2 cm. and 4.1 cm. 
Plate No. J, on Betida. — Three new rootlets were produced. One 
of them became a mycorhiza, and two of them developed as normal 
roots with lengths of 2.5 cm. and 4.9 cm. 
Plate No. 6, on Carya. — Three new rootlets were produced and all 
of them became mycorhizas. 
5. Development of a Mycorhiza 
Apparently the development of a mycorhiza takes place, in the 
early stages, very rapidly. Probably only a day or two is needed for 
the formation of a complete mantle and the development of more or 
less of the "lichen structure," although a considerably longer time is 
required before the fullest development of the mycorhiza is reached. 
