THE ROLE OF GREEN PLANTS 241 



sudh as are seen in the elm, oak, or sassafras; stems with "open" 

 vascular bundles which usually appear as a ring of growing tissue; 

 and seeds with two cotyledons or seed leaves. These structures will 

 be referred to in more detail later. 



The Raw Food Materials Used by Plants 



For a good many centuries after the time of the Greek philosophers 

 who first hold this theory, it was thought that green plants absorbed 

 food from the soil, but it was not until the time of the Belgian philoso- 

 pher van Helmont, who lived in the sixteenth century (1577-1644), 

 that it was clear that water played a very important part in the 

 growth of a plant. One of van Helmont's experiments consisted of 

 placing a willow slip weighing five pounds in a vessel containing two 

 hundred pounds of dried soil. For five years he watered the tree with 

 distilled water, making careful observations on it until it had grown 

 to be a sapling weighing one hundred and sixty-nine pounds and three 

 ounces. But when he weighed the soil in which the tree had grown, 

 he found it had lost only two ounces. Clearly then, the gain came 

 largely from sources other than soil, and he rightly concluded that 

 water was largely responsible for the increase in weight. In the first 

 half of the eighteenth century, an English clergyman, Stephen Hales, 

 worked out the daily water consumption of a plant by ascertaining 

 the relation between leaf and stem surface and the quantity of water 

 absorbed. He went a step further than van Helmont in suggesting 

 that plants take something from the air as well as the soil with which 

 to build up their body material. In 1779, Ingen-Housz, a Dutch phy- 

 sician, who was a co-worker with the famous surgeon, John Hunter, 

 showed that the green part of a plant, when exposed to light, uses the 

 free carbon dioxide of the atmosphere, but that it does not have this 

 power when kept in darkness. A little later, in 1804, de Saussure, 

 by a series of experiments, proved that carbon dioxide and water 

 were both used by plants in the sunlight and that as carbon dioxide 

 was taken from the atmosphere, about the same amount of oxygen 

 was returned to it. He, however, missed the use of the green coloring 

 matter of the leaf in its connection with the sun's energy in building 

 living matter and food. The real explanation of the function of this 

 green substance (chlorophyll) was left for Julius von Sachs, a famous 

 botanist of the nineteenth century. He was the first investigator to 

 demonstrate the fact that green plants make food for the world. Just 

 how they do this is still not fully known, although plant physiologists 



