MOVEMENTS OF GROWTH. 399 



nevertheless, endeavour to make ourselves acquainted with the 

 important phenomena in question. For the experiments we em- 

 ploy seedlings. The seeds from which these are to be obtained 

 must be carefully selected. We employ only seeds which are 

 uniform, well developed, and quite ripe. We shall find that these 

 germinate well if the temperature is favourable (about 20-25 C.) ; 

 on the other hand, we observe at once that the power of germina- 

 tion is no longer so perfect at comparatively low or comparatively 

 high temperatures. The seeds (we experiment with Pisum, 

 Phaseolus, Zea, Cucurbita, etc.) are first laid for twenty-four hours 

 in water, so as to get thoroughly soaked. We now place them in 

 moist garden earth, in such a position that, when their main roots 

 emerge, they can grow straight downwards without needing to 

 undergo any considerable amount of curvature. The soil used is 

 humous garden soil, such as is employed in the cultivation of 

 greenhouae plants. Before being used it is moistened sufficiently 

 to allow of its being rubbed down between the hands into a finely 

 crumbled mass, riddled through a sieve with 1*5 mm. meshes, and 

 then loosely filled into large flower-pots. Finally, the soaked 

 seeds are sown at measured distances from one another, and 

 covered with earth, particular care being taken that they all 

 receive as nearly as possible the same thickness of covering. Each 

 flower-pot is provided with a thermometer, which indicates the 

 temperature of the layer of soil in which the germinating seeds 

 lie. Care also must be taken to replace the water lost by 

 evaporation. The vessels in which the seeds are placed to soak, 

 as also the flower-pots in which the seeds are sown, must be ex- 

 posed to the particular temperatures whose influence on growth it 

 is desired to determine. If we wish to experiment at temperatures 

 of 25, 30, 35, 40, or 45 C., we are obliged to place the culture 

 vessels in thermostats, in which the desired temperature can be 

 maintained. We use, e.g., the apparatus depicted in Fig. 76. 

 Observations at 5, 10, 15, 20 C. are often best made without 

 using thermostats, in suitable places unheated, or warmed by good 

 stoves (in summer, e g. in rooms with a north aspect, or in cellars). 

 But in the course of the twenty-four hours by no means incon- 

 siderable variations of temperature may take place. Particular 

 attention must be paid to this, and the temperature of the medium 

 in which the germinating seeds are situated must hence be con- 

 trolled several times in the course of the day. This must 

 naturally also be seen to in using the thermostats. All the 



