34 SEEDS AND SEEDLINGS. 



a bell-jar or a dry cloth, and compare readings of thermometers at 

 the start of the experiment and then at intervals of a few hours. A 

 rise of 2 C. or more may be observed. 



(b) Instead of jars or tumblers use three test-tubes, each with a bored 

 cork for the thermometer ; place the tubes horizontally in a cardboard 

 box, with thermometer-stems projecting through holes, and pack round 

 and between the tubes with dry cotton-wool. 



59. Influence of Depth of Sowing on Germination. 



In the lower as compared with the upper layers of the soil 

 we find, as might be expected, less air, less circulation of air, 

 and more water. How does the depth at which a seed is 

 placed in the soil affect its germination ? 



* Plant seeds in your glass-sided box at different depths. Put the 

 seeds close to the glass, and (unless the glass has been fixed in with 

 putty so as to be air-tight) put them near the middle of the glass. If 

 the seed is buried too deeply it does not get enough air for healthy 

 germination, and it is apt to be attacked by moulds. 



60. The Atmosphere. As the life-processes of plants 

 depend upon a supply of air, and are greatly influenced by 

 changes in it, the properties and composition of air must be 

 thoroughly understood by the student of plant life. Study 

 this subject, as far as possible, by experimental methods, and 

 consult books on Chemistry, Physics, and Physiography on 

 the following points : 



The weight of air ; pressure of the atmosphere ; Torricelli's experi- 

 ment ; barometers ; variations in air-pressure, and their connexion 

 with weather changes ; effects of pressure and temperature on volume 

 of air ; wind ; gaseous diffusion ; diffusion of gases through small 

 openings (e.g. porous pots) ; composition of the atmosphere ; prepara- 

 tion and properties of oxygen ; nitrogen, nitric acid, nitrates ; action of 

 oxygen on metals and non-metals ; acids, alkalis, salts, oxides ; the 

 differences between a compound, an element, and a mixture ; how air 

 is changed by the burning of a candle ; the presence of carbon dioxide 

 and of water vapour in air ; humidity ; hygrometry ; dew, rain, snow ; 

 cooling caused by evaporation ; conditions favourable to evaporation. 



61. Warmth required for Germination. When the 

 temperature of the soil falls to C. nearly all life-processes 

 become dormant, and for most cultivated plants these pro- 

 cesses do not begin until a temperature above 5 C. has been 

 reached. There is for all plants a certain range of tempera- 

 ture within which germination is most rapid and which 



