ACTION OF LIGHT ON VEGETATION. 659 



box is so arranged that the upper rim of the double wall is on a level with the arm b of 

 the microscope. The opening of the box is closed by a thick cardboard cover d d, in 

 which an opening is cut exactly to fit the arm b. By the side of the tube of the micro^ 

 scope a round hole is cut in the cover through which a closely fitted small thermometer 

 t is passed, so that its bulb hangs near the object. The box is painted on the inside 

 with black varnish, and a piece of cardboard moistened with water lies beneath the foot 

 of the microscope in order to prevent its moving and to keep the air within moist. 

 The focus is easily adjusted to the object by means of the fine adjustment s which pro- 

 jects above the cover; two openings in the side, one of which is shown at 0, enable 

 the slide bearing the object to be moved, when necessary, by a pair of forceps. It is 

 still more convenient to fix the slide on a wire which goes through a cork fitted to the 

 opening 0. 



If observations are required at a higher temperature, the water in the box is heated 

 by a spirit-lamp placed underneath. When the temperature has reached nearly the de- 

 sired point, the spirit-lamp is replaced by an oil-lamp with a floating light ; the temper- 

 ature will after a time become constant. In order to obtain higher or lower constant 

 temperatures, one two or three floating night-lights are placed in the lamp. If care is 

 taken that the combustion be uniform, the temperature in the box remains for several 

 hours so constant that it will vary only about i" C. This constancy of temperature en- 

 sures that the temperature of the object itself is that indicated by the thermometer. 



It is easy by means of this heating apparatus to observe and demonstrate the in- 

 fluence of temperature on protoplasm-currents. To take observations at lower temper- 

 atures it is sufficient to enlarge the hole /, in order from time to time to place pieces 

 of ice in the cold water\ 



Sect. 8. — Action of Light on Vegetation^. A. General. The entire life 

 of the plant depends on the action of light on the cells that contain chlorophyll, this 

 being the essential condition under which new organic compounds are formed out 

 of the elements of carbon dioxide and water. The amount of oxygen evolved in 

 this process is nearly the same as that required for the combustion of the substance 

 of the plant ; and the amount of work equivalent to the heat produced by this com- 

 bustion gives a measure for the amount of work performed by light in the chloro- 

 phyll-containing cells of the plant. 



After a certain quantity of assimilated substance has been produced under the 

 influence of light, a long series of vegetative processes may be carried on at its ex- 

 pense without any further direct action of light. The growth of new organs and 

 the metastasis connected with it kept up in the organs by means of respiration is 

 entirely' or to a certain extent independent of light, and can even be carried on in 

 absolute darkness. This is the case in the germination of seeds, bulbs, and tubers, 

 the development of buds from woody branches and underground rhizomes, &c. 

 Even leafy plants which have accumulated a sufficient quantity of reserve-material in 

 the light, put out shoots and even flowers and fruits when placed in the dark. 



As the parts of chlorophyll-containing plants which are underground or other- 



1 [For further arrangements for maintaining a constant temperature under the microscope, see 

 Strieker and Burdon-Sanderson, Quart. Journ. Micr. Sci. 1870 ; Schafer, ibid. 1874-— Ed.] 



2 A. P. De Candolle, Physiologic vegetale, i832.-Sachs, Ueber den Einfluss des Tages-hchtes 

 auf Neubildung u. Entfaltung verschiedener Pflanzenorgane ; Bot. Zeit. 1863, Supplement.-Sachs, 

 Wirkung desLichtes auf die Bluthenbildung u. Vermittlung der Laubbl^tter ; Bot. Zeit. 1865, p 117. 

 — Sachs, Handb. der Exp.-Phys. 186^, p. i. 



U U 2 



