200 
THIRTEENTH REPORT. 
CONDITIONS WHICH AFFECT THE BRANCHING OF ROOTS. 
LULU M. NEWLON. 
It is a well-known fact that when a root is injured at its tip, so that 
apical growth can no longer continue, its production of lateral roots is 
increased. Roots commonly produce lateral branches, however, while 
their tips are still uninjured and growing. The aerial roots of Philo- 
dendron and vanilla, and the stilt roots of Indian corn, when uninjured, 
do not produce branches while growing in the air, but after entering the 
soil they branch freely. Pond, in his experiments with Ranunculus 
aquatilis, has shown that light inhibits the growth of lateral roots. 
These observations have led to some speculation concerning the condi- 
tions which affect the branching of roots in different plants. Accord- 
ingly, experiments have been carried on by the writer, under the direc- 
tion of Professor F. C. Newcombe, since October, 1910, and are still 
in progress for the purpose of determining the nature of these condi- 
tions. 
The plants which have, thus far, been used, are Elodea canadensis, 
Proserpinaca palustris, Ranunculus aquatilis, Radicula nasturtium-aquat- 
icuni, Ludvigia palustris, Bryophyllum calcynum, Tradescantia zegrina, 
Zea mays and Salix nigra. Experiments have also been begun with the 
roots of Philodendron giganteum and Vanilla planifolia, but so recently 
that nothing has yet been attained. 
The above plants were chosen for this work, some, because their roots 
were known not to branch under certain conditions, others, because 
roots could be easily obtained from them; also, these plants furnish 
examples of soil, aquatic and aerial roots. 
From the experiments which have thus far, been carried on it has 
been found (1) that in none of the above plants does contact act as a 
stimulus for the production of lateral roots, (2) that in all the plants 
used, except Elodea canadensis, lateral roots are freely produced in soil, 
(3) that in the aquatic and semi-aquatic plants used, (except Elodea), 
light inhibits the growth of lateral roots, (4) that in the terrestrial 
plants used, lateral branching of roots takes place with equal freedom 
in light and in darkness, when the roots are under water. 
University of Michigan, April 20, 1911. 
