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. 



(/) 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 pudicd) 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 muscipuld) 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 



