120 PLANT PHYSIOLOGY 
cause it to form its multiciliate zoospores. Study their motion. 
Study also zoospores of Ulothrix, Chaetophora or Draparnaldia 
which can often be obtained by bringing these algae into the 
laboratory and leaving them over night in a dish of water. 
Often they will collect at the side of the glass next to the light. 
(f) With sharp scissors cut off as much as possible of the 
mycelium (fungous threads) of Saprolegnia growing on a fly or © 
piece of meat thrown into a dish of algae. Place it in a dish of 
clean water and after a few hours hang a small piece of meat in 
the water at one side of the dish. After a comparatively short 
time the zoospores produced will be found congregated around 
the meat (chemotaxis). 
177. Turgor Movements. Many plant organs change 
their position or become curved by the change in turgor 
of the cells on one or both sides of the organ. Thus at 
the base of the petiole of the leaf of the sensitive plant 
(Mimosa pudica) there is a strongly developed mass 
of thin-walled cells, the pulvinus. When the cells on the 
lower side are turgid the leaf is held out horizontally or 
inclined upward. In response to various stimuli these 
cells suddenly allow their water to escape into the 
intercellular spaces, thus losing their turgor and contract- 
ing considerably. Apparently the cells on the upper 
side of the pulvinus take up this water very quickly, 
thus becoming turgid in their turn. This process takes 
place very rapidly and results in a quick downward 
bending of the leaves. It is by a similar arrangement 
that the two halves of the leaf of the Venus fly-trap 
(Dionaea muscipula) snap together quickly enough to 
catch insects lighting upon them, or that in the case of 
the sundew (Drosera), when an insect is caught by the 
sticky mass on one of the so-called tentacles, the ad- 
jacent ones bend over until they too touch the un- 
fortunate victim and the whole leaf gradually closes in 
on it. The movement of the stamens in the flower of 
barberry (Berberis) is also due to turgor changes as are 
