502 Arthur. — The Movement of 
It is probable, if we consider the facts now in our possession, 
that in order for streaming to take place the filaments must 
have a high internal pressure. It is doubtless the pressure 
which forces water through places of less resistance in the 
walls, although the walls throughout are highly permeable, 
as shown by the rapidity with which the mycelium collapses 
upon removal from a moist to a dry atmosphere. The ready 
permeability of the walls will account for Wilson’s results in 
obtaining drops by placing particles of sugar on the surface 
of Pilobolus. Internal pressure, as we know, is secured through 
osmotic action, and altogether it seems probable that the 
necessity for a very moist atmosphere to bring about stream- 
ing lies in the fact that the part of the mycelium in contact 
with the moist substratum cannot take up water fast enough 
to secure pressure throughout the whole structure so long as 
extravasated moisture is rapidly removed from the aerial 
surface of the very permeable hyphal walls. 
By admitting the existence of a high osmotic pressure, we 
shall be able to explain many of the phenomena connected 
with the streaming. In the first place we must premise that 
although osmotic pressure originates in the movement of the 
molecules of the osmotic substance and varies directly as the 
density of the solution, yet it is converted into, and is mani- 
fested as, hydrostatic pressure which is uniformly distributed 
per unit area throughout the enclosing wall. The more tensely 
the cell-wall is stretched by the internal pressure, the more 
sensitive every part becomes to any variation in the pressure. 
If we imagine a hyphal tube some millimetres in length, under 
strong pressure, with one end in a watery substratum and the 
other in air, it is easy to see that any water taken up 
osmotically at the submerged end must instantly expand 
some part of the tube (growth), or force water out through 
the walls of the aerial part (extravasation). The water taken 
in at one end and discharged at the other displaces the whole 
column and moves it along toward the end where the water 
is escaping. But in the hyphal tube the water is an interrupted 
column lying in the more viscous protoplasm, and the move- 
