26 Principles of Plant Culture. 
latter down closely over the beans, cover both saucers with 
a bell glass, and place in a warm room for two or three days, 
we shall find that the beans covered with the sand will 
sprout promptly, while those covered with the puddled soil 
will not (Fig. 4). In the sand-covered saucer the air has had 
Fic. 4. In the left hand saucer, beans were planted in puddled soil. In the 
other, they were covered with sand. They failed to germinate in the puddled soil, 
because their contact with oxygen was cut off. (From nature). 
access to the beans between the grains of sand, while in the 
other the“air has been shut out, which explains the sprout- 
ing of one lot of seeds and the failure of the other. About. 
one-fifth of the air is free oxygen, i. e., oxygen that is not 
chemically combined with any other substance. 
We have seen (13) that protoplasm in its active ae 
requires oxygen. Unless seeds are so planted that a certain 
amount of this free oxygen can reach them they cannot 
germinate.* Ordinary water contains a little free oxygen, 
but not enough to enable many kinds of seeds to germinate 
in it, though the seeds of some water plants, as the water 
lily and rice will germinate in water. But even these will 
not germinate in water that has been boiled long enough to 
expel the oxygen, and placed under conditions that prevent 
the absorption of it again. 
We thus see that seeds require three conditions before 
they can germinate, viz.,a certain amount of moisture, of 
* This probably explains why very deeply planted seeds rarely germinate. 
