30 HUMAN PHYSIOLOGY 



he reviewed a certain memorable experiment made about 

 three hundred years ago. He had planted a young shoot 

 of willow in a tub of earth and let it grow for five years. 

 It had then increased in weight by about 160 pounds, 

 though the soil had lost only a few ounces. So far as 

 Van Helmont could judge the tree must have been made 

 from the water that had been freely supplied. Water 

 had indeed entered into its structure, but a larger addi- 

 tion was due to the unrecognized carbon dioxid of the 

 atmosphere. 



Interrelations of Plants and Animals. It is now evi- 

 dent that a certain reciprocity exists between plants and 

 animals. But this is true only when the plants consid- 

 ered are those with pigment; the uncolored varieties 

 fungi, etc. are unable to make the fuels which they con- 

 sume and are, therefore, like the animals in their depend- 

 ence on the higher plants. Some forms exist which have 

 intermediate powers. It is to be borne in mind, that, 

 while the differences between typical animals and plants 

 such as we naturally choose for comparison are striking 

 enough, there are numerous species low down in the scale 

 which are not surely to be assigned to one class or the 

 other. Some of these have been claimed alternately by 

 the botanists and the zoologists. 



Excluding all but the most highly developed types, let 

 us consider how the proximity of plants and animals in- 

 fluences the economy of the two. A specific illustration 

 may be suggested. Suppose that a snail is living in a hot- 

 bed. The animal eats portions of the plants, living or 

 dead. It returns carbon dioxid and water to the air 

 and soil of the enclosure. The green leaves, transmuting 

 the energy of light rays, can recover the fuel which the 

 animal has dissipated. In thus compensating for the 

 spendthrift proclivities of the animal the plants have 

 returned to the air the oxygen which the animal appropri- 

 ated. It is clear that the plants are necessary to the 

 animal and that they must grow fast enough to make up 



