Again, there is a substance called cellulose, found well nigh universally in plants.  Of this substance, which is akin to starch, the walls or envelopes of the cells of plant tissues are composed.  Yet we find those curious animals, the sea squirts, found on rocks and stones at low-water mark, manufacturing cellulose to form part and parcel of the outer covering of their sac-like bodies.  Here it is as if the animal, like a dishonest manufacturer, had infringed the patent rights of the plant.  On the fourth count, then—­that of chemical composition—­the verdict is that nothing that chemistry can teach us may serve definitely, clearly, and exactly to set a boundary line or to erect a partition wall between the two worlds of life.  There yet remains for us to consider a fifth head—­that of the food.

In the matter of the feeding of the two great living worlds we might perchance light upon some adequate grounds for making up the one kingdom from the other.  What the consideration of form, movement, chemical composition, and microscopic structure could not effect for us in this way, it might be supposed the investigation of the diet of animals and plants would render clear.  Our hopes of distinguishing the one group from the other by reference to the food on which animals and plants subsist are, however, dashed to the ground; and the diet question leaves us, therefore, when it has been discussed, in the same quandary as before.

Nevertheless, it is an interesting story, this of the nutrition of animals and plants.  A large amount of scientific information is to be gleaned from such a study, which may very well be commenced by our having regard to the matters on which a green plant feeds.  I emphasize the word “green,” because it so happens that when a plant has no chlorophyl (as green color is named in the plant world) its feeding is of diverse kind to that which a green plant exhibits.  The mushroom or other fungus may be taken as an illustration of a plant which represents the non-green race, while every common plant, from a bit of grass to an oak tree, exemplifies the green-bearing order of the vegetable tribes.

Suppose we were to invite a green plant to dinner, the menu would have to be very differently arranged from that which would satisfy a human or other animal guest.  The soup would be represented for the plant’s delectation by water, the fish by minerals, the joint by carbonic acid gas, and the dessert by ammonia.  On these four items a green plant feeds, out of them it builds up its living frame.  Note that its diet is of inorganic or non-living matter.  It derives its sustenance from soil and air, yet out of these lifeless matters the green plant elaborates and manufactures its living matter, or protoplasm.  It is a more wonderful organism than the animal, for while the latter can only make new protoplasm when living matter is included in its food supply, the green plant, by the exercise of its vital chemistry, can transform that which is not living into that which is life-possessing.