CULTIVATION TO CONTROL GROWTH OF YOUNG PLANTS 



Hard rains sometimes pack the soil, limiting the air supply. Cultivation loosens and 

 aerates the soil. It also limits the loss of water by evaporation. Cultivating beans 

 at the time the "necks" are pulling the seed leaves above the ground may break off 

 and kill many of the young plants 



It may be that other factors also play a part after all. For example, in the 

 presence of water seeds may sprout at one temperature but not at another. 

 From actual experience we know that we may safely sow seeds of some 

 species earlier in the spring than others. From experiments we learn also 

 that some seeds will fail to sprout when it is too cold or too warm. 



How Is Air Related to Life? 



Air and Life The atmosphere has approximately the composition 

 shown by the diagram in the illustration on page 84. When air is shut off, 

 we suffocate, as in drowning. Now what is the connection between air and 

 being alive? 



The energy of protoplasm, in all its activities, comes from the burning, or 

 oxidation^ of materials derived from food. The food is not burned directly, 

 like the oil in a furnace. It hrst undergoes many changes through which it 

 is finally assimilated, or made into living protoplasm. Nor is the oxidation, 

 or burning, like the familiar flame. It takes place only in the presence of 

 water, whereas the fires with which we are familiar cannot burn under water. 



83 



