GERMINATION, GROWTH, TISSUE TENSION. 155 



single plant to various degrees of temperature for equal periods of 

 time, keeping all other conditions constant ; this is done either by 

 using a single thermostat and altering the temperature at intervals, 

 or by transferring the plant from one thermostat to another at a 

 different temperature. (2) We may expose a series of similar plants 

 to different degrees of temperature ; this is done by using a diffe- 

 rential thermostat, consisting of a series of chambers cooled (by ice 

 or by circulating water) at one end and heated at the other, one 

 plant being placed in each chamber and the temperatures of the 

 chambers ranging from, say, 5 to 60 C. 



As a rough experiment, sow a number of seeds of the same kind 

 in a series of three or four pots, giving equal light, air, and water to 

 each. Place the pots in different positions known to vary in tempe- 

 rature, in one of the following ways : 



(a) Place some soaked seeds in a glass jar and cover them with 

 moist sawdust ; plunge the jar into a box containing pieces of ice, 

 which must be renewed as they melt. The ice will last longer if the 

 box containing it is set into a larger box, and the space between the 

 two boxes is packed with dry sawdust (why ?). 



(6) Another method is to use two boxes as in the preceding, but to 

 place in the smaller box a single bit of ice, with dry sawdust below 

 and around it ; place the seeds directly on the ice and cover them 

 with dry sawdust, which will be kept moist by the melting ice. 



(c) In winter and spring the minimum temperature for germina- 

 tion should be determined for as many seeds as possible. Into a 

 large flower-pot or seed-pan put some bits of broken earthenware at 

 bottom, and fill up the rest of the pot with sifted soil. Plant in the 

 pot a few seeds of different kinds, and bury the bulb of a thermo- 

 meter at the depth of the seeds, tying the thermometer stem to a 

 stick thrust into the soil. Sink the pot up to its rim in the soil of a 

 garden bed and record the temperature each day, looking for any 

 signs of germination. After two or three weeks bring the plants 

 indoors ; keep the soil moist ; make notes of your observations. 

 Other pots should be kept in different parts of the house or school, 

 in addition to those kept outside. Such experiments will show that 

 warmth hastens germination, while cold retards it. 



205. Growth dependent on Oxygen. (1) Soak six 

 Peas in water, and let them germinate until the root is 

 about 1 cm. long. Measure the length of the root of each 

 seedling from an ink mark on one cotyledon, then pass 

 three of the Peas (A) up into an inverted test-tube of 

 mercury, as in the experiment on intra-molecular respira- 

 tion. Place the other three Peas (B) in wet sawdust or 

 sphagnum. After a day or two, measure the roots again 

 and note that in A very little growth has occurred. 



