60 PLANT PHYSIOLOGY 



measurements, reduced to millimeters per minute, the rate of the 

 swiftest streaming at the room temperature, which is to be noted. 

 The results should be expressed not only in words and figures, but in 

 drawings representing diagrammatically in all three dimensions the 

 paths and extent of the streaming. 



PRECAUTIONS. To ensure normal streaming, one must take care not 

 to give the material any sudden shock, either through rough handling, injury 

 by forceps, pressure on cover-glass, or sudden transitions in temperature. 

 It is best to mount the material and let it stand for some time, a quarter of 

 an hour or more, before using, in the case of Nitetta, the material, if prevented 

 from drying up by use of a small moist-chamber made with filter-paper 

 and a tight glass dish, may be kept in perfect condition for hours or even 

 days. Enough water must be used in the mount to float up the cover-glass 

 and prevent it from pressing on the specimen. Observation of the first 

 specimen seen is to be shunned; a number should be looked over, and typ- 

 ical ones, preferably several, should be selected, and the mean of their rates 

 determined. The lowest magnification and longest spaces of movement 

 practicable give most accurate results. Other precautions are given in 

 EWART'S " Protoplasmic Streaming," 24, 53; and the student should espe- 

 cially be on guard against the various sources of error, as discussed earlier 

 (page 14) in this book. 



MICROSCOPICAL MEASUREMENTS. These can be made approximately 

 by determining first, with a stage micrometer, the diameter of the field of 

 vision, and then noting the time requisite for certain granules to cross a 

 given fraction of the field. But far more accuracy is attained by the use 

 of an ocular micrometer (a scale ruled on a glass disc which can be placed 

 on the diaphragm of the eyepiece), which is clearly in focus with the mov- 

 ing Protoplasm. The value of the spaces in millimeters depends upon the 

 magnifying power of the combination with which it is used, and while this 

 may be stated by the maker, it is much better to determine it anew for each 

 combination by comparison with a stage micrometer ruled in small fractions 

 of a millimeter. Then the time requisite for streaming a given distance* 

 is readily determined. The time may be counted by a watch or a clock; 

 but far better is a second-ticking clock or a metronome, the inexpensive 

 instrument used in musical instruction. For very exact work a stop-watch 

 would no doubt be best of all. 



PROTOPLASMIC-STREAMING QUANTITIES. The rate of streaming varies 

 from near o to 10 mm. per minute, though the latter is very exceptional; 

 for most plants the optimum rate lies between .3 and i mm. per minute, 

 and may conventionally (page 22) be taken as .7, thus making the conventional 

 expression o-.y-io mm. per minute. The rate is correlated with tempera- 

 ture, rising from a minimum degree of no movement up to an optimum 

 degree of greatest rate, whence it sinks again to a maximum degree of no 

 movement. These three points vary with different plants, but approxi- 

 mate to, and may conventionally be expressed as, 5, 35, and 45. 



Graphs showing the relation of temperature to rate of streaming are 



