48 Practical Plant Biology. 



measurements of the length and width of the cell and the size of the chloro- 

 plasts and nucleus by means of the Ghost-micrometer in a thin slice of the 

 leaf. 



Make a careful drawing of the cell, putting in all the structures you have 

 observed. By careful focussing try and picture to yourself what would be 

 the appearance of a cross section of this cell. Make a drawing of your re- 

 construction. 



Sooner or later while making these observations, the attention of the ob- 

 server will be arrested by the surprising fact that the chloroplasts of some, or 

 of all the cells, are moving. 



It will be apparent that their motion is due to the motion of the cytoplasm 

 of the cells, which carries in its streaming motion all the substances em- 

 bedded in it around the cell. Minute observation shows that the immedi- 

 ate surface of the protoplasm next the cell-wall does not move, while the 

 streaming affects the deeper layers. At first when a leaf is pulled off a plant 

 and mounted, the cytoplasm is generally quiescent and stationary in the cells, 

 but usually after a few minutes of freedom from interference, motion gradually 

 reappears. The first arrest of the motion is attributed to shock. As a rule 

 motion may first be detected in some of the long cells of the mid-rib. At 

 first it is very slow and appears almost tentative : a single chloroplast or 

 group is slightly moved, then the motion extends to a larger piece of the 

 cytoplasm and soon a stream is started which carries the chloroplasts and 

 nucleus slowly up one side of the cell and down the opposite side. The 

 streaming quickens its pace and soon attains a maximum. While these 

 events have been taking place in one cell, similar phenomena may be seen 

 starting in the neighbouring cells, and soon in all or nearly all of the cells of 

 the leaf this streaming motion is in evidence so that the field of the micro- 

 scope presents a striking and busy appearance. When the streaming is 

 sluggish it may often be accelerated by gently warming the preparation. 

 A temperature which kills the protoplasm stops the streaming. It is a 

 phenomenon of life and is a good example of one of the so-called spontaneous 

 actions of protoplasm. So far no satisfactory mechanical explanation of it 

 has been given. But it is certain that to maintain it an expenditure of energy 

 is needed. Work must be done to overcome the viscosity of the protoplasm. 

 Thus this motion is of special interest as being one of the visible processes of 

 the cell which consumes some of the energy acquired by the plant. 



Streaming may be observed in the cells of many plants but it is seldom so 

 marked as in the example before us. Usually the displacement of the various 

 cell contents is very slow and imperceptible under the ordinary conditions of 

 observation. 



