“I spoke of carbonic acid; well, this is a gas, as some of you have found out before, made up of carbon and oxygen.  It is a gas which we of the animal kingdom breathe out as waste from our bodies.  The plant takes it in through the leaf—­and, by the way, I ought to explain that.  It is this way:  if we had a magnifying glass we should find over the inner surface of leaves, pores, or stomata as they are called.  They open in the presence of light; and from these openings what the plant has no use for passes out, and gases from the air may pass in.  Some call these openings breathing pores.

“Quantities of water pass out through these pores.  When this process goes on too rapidly a plant will wilt.

“So, to go back, we will suppose that carbonic acid gas has passed into the leaves.  Straightway the chlorophyll bodies get to work.  The gas is broken up, and oxygen and carbon are left.  The carbon is wood the plant builds.  Some of the oxygen passes out into the air and some is kept for plant food use.

“It is a good thing for us that some of the oxygen does escape into the air for we need it.  So you see we, in our respiration, and the plant, in its breathing, are doing each other a good turn.

“Of course, there is the dilute food from the soil, which is largely mineral matter and water.  The chlorophyll bodies work away on these minerals, and make them into foods.  A great body of water, as I have said before, passes out of the plant through the stomata.

“I have told you a thing that the plant can do which we are not capable of doing.  A plant takes a mineral and makes it over into food.  You and I, unless we happen to be circus glass-eaters, are not built to do this work.  But the vegetables which we eat do the work for us.

“A great deal of plant food is in the form of sugars and starches.  I remember Katharine and Peter told me last winter that in their physiology they learned how sugars and starches were made in our own bodies.  And lo and behold, the geranium can do a similar thing.

“Some plants store up lots of starch, as the potato.  Others store quantities of sugar, as the Southern sugar cane and the beet.  Wonderful?  Well, I guess it is.  If we could hear and see all the work these energetic little chlorophyll bodies are doing, we should be amazed.

“You will remember that I told you some plants could take the very necessary chemical nitrogen from the air; most of them, however, must get it from the soil.  And so again this from the soil solution is worked over into available food.

“After all we must not fail to see that water is most important.  It floats all the important food elements to the leaves for the work to be done there.  The food carbon, of course, is an exception to this rule and I will say again in certain cases nitrogen is, also.

“Thus you boys and girls now understand how necessary it is that a soil should be of the right texture to hold water.  If it is not, it must be helped to be so.  Sand, you will remember, had to be doctored to hold water.  Clay needed treatment in order to make it quit its bad habit of baking out.