250 
Four species of rust fungi were investi- 
gated. In the case of Uromyces euphorbic 
Cooke & Peck, the well-known rust of Eu- 
phorbia, it was demonstrated by three sepa- 
rate series of experiments that the rust is 
able to propagate itself constantly through 
the germinating seed of its host, and there- 
fore becomes in that way practically a per- 
ennial species. It is the only demonstrated 
example of this manner of propagation in 
the whole order of Uredineae. Actual clus- 
ter cups may be seen in the hulled seeds of 
Huphorbia dentata. Seedlings kept under 
‘bell jars become rusted three months from 
the date of planting, showing all stages of 
the rust, while seeds disinfected with mer- 
curic chloride produce no rusted plants. 
Culture experiments were also performed 
with the common sunflower rust, which 
showed that the Puccinia and Beidium 
found on sunflower are stages of one and 
the same species. At the same time it is 
made probable that all the species of Heli- 
anthus affected bear the same rust and 
that there is no distinction of host forms. 
The peculiar, thick-walled, one-celled spores 
of Puecinia vexans Farl., have at last been 
successfully germinated after repeated fail- 
ures,and it is nowseen that these spores 
are neither properly uredospores nor teleu- 
tospores, but partake of the nature of both. 
They make up a distinct new spore form 
for this order of fungi, and may be called 
amphispores. True uredospores were also 
found and germinated. Other experiments 
and observations have shown that Meidiwm 
tuberculatum Ell. & Kell. is commonly a 
perennial species in its perennial host Cal- 
lirrhoe involucrata, producing spores able to 
germinate during the coldest winters. 
Rheotropism of Roots: PRorESsOR FREDERICK 
C. Nrewcomse, University of Michigan. 
The phenomenon of rheotropism is mani- 
fested by a curvature of the root when grow- 
ing instreaming water. In all cases so far 
SCIENCE. 
[N. S. Vou. XIII No. 320. 
observed the response has been positive, i. e., 
the root-tip curves against the stream. 
The present research has included 32 
species of plants, of which 15 have shown 
themselves rheotropie and 17 insensitive. 
Nearly related plants behave similarly ; 
but of two genera of the same family, one 
may respond to the current of water, and 
the other may be insensitive. Species dif- 
fer greatly in degree of response. Members 
of the Crucifere are among the most sensi- 
tive plants found, their roots often attain- 
ing an angle of 90° from the vertical. 
The velocity of current calling forth the 
best response lies between 100 em. and 500 
cm. per minute. A velocity of 2,000 cm. 
per minute will in most plants bring a 
mechanically negative curve, and the re- 
sponses in currents less than 100 em. per 
minute are weak and transitory. However, 
a velocity as low as one cm. per minute will 
bring a curvature in the majority of roots 
of the garden radish. 
The latent period at the optimum tem- 
perature for growth is one hour or more. 
The area which perceives the stimulus 
includes the apex of the root and the elon- 
gating zone. 
Roots of mature plants as wellas those of 
seedlings are responsive. 
The author four years ago suggested that 
the stimulus might really be referred to the 
one-sided pressure of the water upon the 
root. Considerable evidence is now offered 
to confirm this view. 
Thigmotropism of Roots: PRrorgssor FRED- 
ERICK C. NewcomBs, University of 
Michigan. 
Only two authors have claimed for ordi- 
nary roots the presence of sensitiveness to 
contact or pressure. Darwin believed he 
had found a negative response (a turning 
away), when the sloping side of the root 
apex touched a foreign body; and Sachs in 
a single and simple experiment found some 
