ACTION OF LIGHT ON VEGETATION. 



737 



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 o, 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 o. 



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

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

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

 perature 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 

 ensures 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 influence 

 of temperature on protoplasm-currents. To take observations at low temperatures 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 of 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 Hght, a long series of vegetative processes may be carried on at its 

 expense 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 Hght, 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- 

 wise excluded from light are nourished by the products of assimilation produced in 

 the light, so also parasites and saprophytes destitute of chlorophyll live, as has 

 already been explained, on the work performed by plants that contain chlorophyll, 

 and are therefore dependent indirectly on light, even though the whole of their 

 development may be completed in darkness, as in the Truffle ; in other instances 

 they only emerge to unfold in the air the flowers already formed underground, and to 



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

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



2 A. P. De Candolle, Physiologic vegetale, 1832.— Sachs, Ueber den Einfluss des Tages-lichtes 

 auf Neubildung u. Entfaltung verschiedener Pflanzenorgane ; Bot. Zeitg. 1863, Supplement. — Sachs, 

 Wirkung des Lichtes auf die Bliithenbildung u. Vermittlung der Laubblatter; Bot. Zeitg. 1865, 

 p. 117. — Sachs, Handb. der Exp.-Phys. 1865, p. i. 



3 B 



