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68 MISSOURI BOTANICAL GARDEN. 
branches is without doubt due to a stimulus exerted by the 
fungus. The only causes which could bring about advance- 
ment toward the root tips of lateral root-formation are two, 
pressure at the tip, and injury to the same, both equally 
effective in preventing onward growth of the root.* The 
experiments of Pfeffer and of Newcombe showed that 
growth had to cease entirely before the lateral roots make 
their appearance. Experiments of my own led to similar 
conclusions. Seedling beans, peas, cotton, squash, castor 
beans were grown in plaster, both completely hard, and 
softened plaster kept in moist chambers, solutions of gel- 
atin, in rubber bags, etc. The results are to be recorded 
elsewhere. All the experiments showed conclusively that 
only when there was an absolute cessation of growth, did 
the lateral roots appear. The pressure must be a great one, 
which prevents onward growth, and it is scarcely conceiv- 
able that the pressure exerted by an envelope of fungus 
hyphae would be sufficiently great to do this. But aside 
from this the pressure, if such existed, did not stop the 
growth, for as has been said, the roots all kept on growing, 
even after being covered by the fungus sheath. It would 
then appear that the only other cause except that of stimu- 
lus, which would bring about lateral root development, 
could not have been effective in this case. 
It is impossible to say what kind of fungus it was, which 
enveloped the roots, but it seems probable that it is dis- 
tinct from those forming the mycorhiza. It is by no 
means a rare occurrence to find fungus mycelium about 
beech roots, which seems to have no connection with the 
same. Masses of roots collected at Oran, Mo., during Octo- 
ber were almost white in color because of some fungus 
surrounding the roots like a fine web. The smaller roots 
* Pfeffer, W. Druck und Arbeitsleistung durch wachsende Pflan- 
zen. (Abh. d. kénig. siichs. Ges. d. Wiss. 20: 235-474. 1893.) —New- 
combe, F. C. The influence of Mechanical Resistance on the Develop- 
ment and Life Period of Cells. (Bot. Gazette 19: 149. 1894.) 
