ON THE PILOBOLID.E. 
215 
manifest, when it is observed that the drops are long main¬ 
tained, or even originated, in situations in which the 
condensation of atmospheric vapour would be impossible, as, 
for instance, while the specimens are being examined beneath 
the microscope in the concentrated beam of the lamp. In 
fact, if a single specimen removed from its matrix be watched 
in such a position, it will be observed speedily to become dry 
and withered ; but if a vigorous individual be taken, with a 
certain quantity of the moist substratum attached, the drops 
will easily maintain themselves, thus showing that the supply 
is kept up from the interior of the Pilobolus stem, which again 
derives it from the mycelium. 
But the liquid is not pure water ; it has a greater refractive 
index, resembling that of the cell sap which occupies the axis 
of the fungus, and it contains dissolved in it the same crystal¬ 
loid substance which we know the cell sap to contain. When 
the drops adorning the exterior of a Pilobolus become dried 
up, they often leave behind on the surface a number of minute, 
angular, transparent, crystal-like bodies, which are apparently 
identical with mucorine. The cause of this excretion of course 
lies in the upward pressure of the contents of the stem, which 
drives a little of the more fluid material through the cell 
walls. 
The dewdrops occur on every part of the stem and swelling, 
and also in the angle between the latter and the sporangium, 
but very rarely on the sporangium itself. Moreover, when an 
individual is placed, as above described, in the concentrated 
beam of the microscope lamp, the sporange, though intact, 
speedily becomes shrunken and the cap puckered and 
depressed, while the swelling retains its fulness if it finds a 
sufficient supply of moisture from beneath. These facts show, 
what we have already found reason for believing, that there is 
very little communication between the sporangium and the 
stem, after the septum is once formed. 
The earlier writers on Pilobolus were much struck with 
this phenomenon, and indeed few objects can be conceived 
more beautiful than a well-grown specimen thus bedecked 
with pearls of dew. One of the species, F. roridus, obtained 
its name from being pre-eminently thus adorned ; but the 
dewdrops are to be met with in every species. And it is not 
a pretty sight only, but, as it seems to me, has a direct 
bearing upon the now important question of the porosity of 
the cell-wall. For the process would appear to be not an 
inter-molecular diffusion, but an actual passage of fluid 
through minute canals piercing the membrane ; and it must 
be remembered that, so long as the supply from beneath is 
