70 THE AMERICAN MONTHLY [Mar 
blue is quite satisfactory. The structures should be 
mounted so as to exclude all air-bubbles; a photomicro- 
graph of delicate tissues in which a large bubble is in- 
corporated is, asa rule, quite spoilt on account of the ef- 
fects of reflection and refraction at the surfaces of the bub- 
ble. Styrax is a good mounting medium, Canada balsam 
being rather yellow even in thin films.—H. A. Haig in 
Science Gossip. 
GroRGE MassExr’s VIEW AS TO THE ORIGIN OF POTATO 
TUBERS.—Until quite recently it was generally supposed 
that potato tubers were included in the general scheme 
of evolution that resulted in the production of the potato 
plant, their function being that of reproducing the species 
in a vegetative or asexual manner. In this supposition, 
from the point of view of Noel Bernard, we have been | 
very much mistaken. According to him, the presence of 
tubers on a potato plant is quite accidental, and entirely 
depends on irritation of the tips of the subterranean 
branches occasioned by a minute fungus called Fusarium 
solavi. This particular fungus is so abundant in the | 
soil that it is practically impossible for potato stems to 
escape the necessary irritation, which results in a tuber. 
To prove this theory the author planted sterilized potato 
tubers in sterilized quartz sand. In some instances no 
tubers were formed; this proved the truth of the theory, 
so says M. Bernard, In other cases a few tubers were 
formed, but according to the author these were due to 
the fungus having gained access to the roots in the sand. 
Many people have planted potatoes under conditions 
much more favorable than that of sterilized quartz sand, 
and yet have failed to secure a crop. Fusarium solani 
- is considered by vegetable pathologists as the fungus 
which causes one of the worst of diseases to which po- 
tatoes are subject. 
The idea of the formation of swellings on the roots of 
et ee ae ——= 
