110 PLANT BIOLOGY 



perature of each place where you put a tumbler. Each day 

 look at the tumblers, and if necessary add enough water to 

 keep the level the same in all three. 



1. Which one of the five conditions named in 118 is different for 



the three groups of seeds ? 



2. Name all the conditions which are practically the same for the 



seeds in all three of the tumblers. 



3. At the end of a few days compare the seeds in the three tumblers. 



What percentage of the seeds in each of the three tumblers 

 has germinated ? 



4. State clearly your conclusion as to the relation of temperature to 



the germination of pea seeds. 



5. Allow the three tumblers to stand in the three different tem- 



peratures for several weeks. Describe the changes that take 

 place in each tumbler, and state your conclusion as to the 

 relation of temperature to the growth of pea plants. 



121. The soil. " From the soil all things come; and 

 into it all things at last return ; and yet it is always new, and 

 fresh, and clean, and always ready for new generations. This 

 soft, thin crust of the earth so infinitesimally thin that it 

 cannot be shown in proper scale on any globe or chart sup- 

 ports all the countless myriads of men, and animals, and 

 plants, and has supported them for countless cycles, and will 

 yet support for other countless cycles. In view of this 

 achievement, it is not strange that we do not yet know the 

 soil and understand it ; and we are in a mood to be patient 

 with our shortcomings." l 



Even a casual examination of the soil in any region shows that 

 it has a complex structure. Usually it is composed of some coarse 

 particles known as gravel, finer grains called sand, and still more 

 minute ingredients, the mud or clay. The relative proportions of 

 these constituents determine whether the soil is a gravelly soil, a 

 sandy soil, or a clayey soil. The soil particles to which we have 



1 Bailey's " Cyclopedia of American Agriculture," Vol. I, " Farms," 

 p. 323. 



