THE GROWTH OF FOREST TREE ROOTS 
MyCORRHIZAL RELATIONS 
In a previous paper (6) I stated that ectotrophic mycorrhizas are 
formed in late summer and autumn. In the Hght of the present in- 
vestigation it would seem that the time of formation would vary 
more or less with the season. Two conditions are necessary for the 
formation of mycorrhizas; the roots must be growing and the proper 
fungus must be present and in an active and receptive condition. In 
the season of 1912, when the above mentioned work on mycorrhizas 
was done, the spring was wet enough for root growth but the early 
summer was very dry, while from the latter part of July on there was 
again plenty of moisture. Since no mycorrhizas were formed in the 
spring it may be supposed that the second condition mentioned above, 
the presence of a suitable fungus, was not fulfilled. So little is at 
present known concerning the ecology of the mushrooms that cause 
mycorrhizas that it is perhaps idle to speculate on their condition and 
activities in the spring of the year, but it is known that mycorrhizas 
are produced largely by the later fruiting mushrooms rather than by 
the spring forms. Since the fruit bodies are usually produced soon 
after the fungi have become attached to roots, it is reasonable to 
suppose that they are not in a condition for mycorrhiza formation 
earlier in the season. 
Three of the species of trees used in the present investigation pro- 
duce ectotrophic mycorrhizas, the oak (Quercus), hickory (Carya) 
and linden (Tilia). The mycorrhizas of the oak are due to Russula 
foetentula Pk., those of the linden to Scleroderma vulgar e, Fr., and those 
of the hickory probably to Laccaria ochropurpurea (Berk) Pk., though 
this last has not been definitely proven. In all of these cases no 
mycorrhizas were formed in the spring, but after the first of July 
mycorrhizas were formed whenever the roots were growing well. 
Conclusions 
1. The root growth of forest trees begins as early in spring as the 
soil becomes warm enough for absorption and ceases in autumn when 
the soil becomes too cold. 
2. There is not necessarily a summer resting period. 
3. When there is a summer resting period it is due to a decrease 
in the water supply and not to any inherent tendency toward perio- 
dicity. 
Botany Department, 
University of Illinois 
