420 PHYSIOLOGY OF GROWTH. 



has gone over into a state of transient heat-rigor. After one to 

 two hours, however, at a lower temperature, protoplasmic move- 

 ment is again exhibited. 5 



Protoplasmic movement is arrested at low as well as at high 

 temperatures. In many cases the temperature at which the proto- 

 plasm, while not yet killed, is still, for the time being, in a state of 

 cold -rigor, is to be sought at 2 or 4 C. If, however, shoots of 

 Cucurbita are kept in a place at a temperature of 10 C. for some 

 time, in fact till they cool down to this temperature, the proto- 

 plasm in the hairs is already in a state of cold-rigor. With rise 

 of temperature the circulation of the protoplasm begins again. 



By means of special apparatus for regulating the temperature, 

 we can determine that with rise of temperature from a minimum, 

 the movement of the protoplasm becomes more active, proceeds 

 most actively at a particular temperature (e-g- about 35 C. in the 

 hairs of Cucurbita), and at temperatures above this again falls 

 off. 



A very useful arrangement of this kind has been constructed by 

 Pfeffer, 6 but Sachs' apparatus, 7 depicted in Fig. 137, may also be 

 employed. The size of the warm chamber must correspond with 

 that of the microscope. The case, approximately cubical inform, 

 has double walls below and at the sides made of sheet zinc, and 

 separated from each other by an interval of 25 mm., the space 

 between them being filled with water. The case is quite open 

 above, but there is an opening in the front wall, which is closed 

 by a well-fitting but not otherwise fixed sheet of glass. The 

 window, D, is of such size, and so situated, as to allow sufficient 

 light to fall on the mirror of the microscope standing within the 

 case. The height of the case is so arranged that the top of the 

 double wall is on a level with the bridge of the microscope, and it 

 is provided with a thick cardboard cover, cut out so as to fit 

 exactly round the bridge. Close to the tube of the microscope 

 there is a round hole in the cover, in which a small thermometer 

 fits tightly, its bulb hanging beside the objective. The box is 

 painted on the inside with black varnish, and a piece of cardboard 

 saturated with water is placed under the foot of the microscope, 

 which is thus made to stand more firmly. The moistened card- 

 board cover also serves the purpose of keeping the air moist round 

 the object under examination. This latter is easily focussed by 

 means of the focussing screw which projects through the cover 

 two side openings, of which the figure shows one, -F, render it 



