and Laboratory Methods. 2569 



Methods in Plant Physiology. 

 XVI. 



(CONCLUDED.) 



CHEMOTAXIS. 



The response of free swimming organisms to chemicals is most conveniently 

 studied by the use of mature antherozoids or motile bacteria. Fern prothalli 

 bearing mature antheridia may usually be found after a little search in any green- 

 house, or they may be grown by sowing spores in moist earth two months in 

 advance of the time when required. Suitable prothalli should be placed upon 

 slides and freed from earth by means of a dry brush and scalpel-blade ; better 

 results usually follow if they are allowed to dry for two or three minutes, then a 

 few drops of rain water or filtered hydrant water may be added and the prepar- 

 ation covered with a cover-glass. If mature antherozoids are present they will 

 be detected in a few minutes with the low powers of the microscope. Some 

 prepared capillary tubes should now be ready to insert under the cover-glass. 



The method of preparing the tubes is as follows : Heat a small glass tube 

 in a flame and draw it out to a slender filament. Break up the thinnest part of 

 the filament into pieces S to 10 mm. long and sketch them under a microscope 

 with the aid of a camera lucida. By means of a stage micrometer measure the 

 internal diameter of the tubes, selecting only those which are between .10 and 

 .14 mm. and fusing up one end of each in a flame. They are then placed in a 

 small dish of .05 per cent, neutral sodium malate (.05 per cent, malic acid neu- 

 tralized with sodium hydroxid) and the air exhausted from them under the 

 receiver of an air-pump. When the tubes are removed from the receiver some 

 of the solution will enter them in place of the removed air. 



Taking one of the tubes in a pair of small forceps, rinse it in water and insert 

 it underneath the cover-glass, directing the open end of the tube to a place where 

 antherozoids are seen swarming about. Note the entrance of the antherozoids 

 into the tubes and whether any of them pass out again. 



The same general method may be followed, using bacteria instead of anthero- 

 zoids. A culture of bacteria may be obtained by boiling a pea for several min- 

 utes to kill it, and allowing it to putrefy in an open dish. Mount a drop of this 

 culture liquid on a slide and insert, under the cover-glass, a capillary tube filled 

 with a 1 per cent, solution of beef extract. Note the subsequent movements of 

 the bacteria as they enter the area where the beef extract is diffusing out from 

 the tubes. Study the reactions which ultimately bring them into the tubes. 



AUTONOMOUS MOVEMENTS. 



1. Circuranutation of Stem Tips. — The circumnutation of rigid stem tips is 

 sometimes very small and a method must be employed which exaggerates the 

 movement. The following method is taken from Darwin's Power of Movement 

 in Plants and has been found to be very practical. 



Place a potted Coleus in the bottom of a tall box which admits light only 



