TRANSLOCATION OF CARBOHYDRATES 257 



storage. He was led to this view by observing that the seed 

 leaves of a vegetable-marrow plant developed into green leaves 

 not very different from the ordinary ones and that if they were 

 cut off the young stem did not grow. Hence he supposed that 

 leaves in general were organs for changing the crude sap brought 

 up from the roots into substances fit for promoting growth. 



But Malpighi's work was frequently overlooked and the older 

 form of the circulation theory continued to prevail. Not until 

 long after his time was a knowledge gained of the processes 

 carried out by green leaves in sunlight ; previous to this their 

 importance in the question could not be realised. At the begin- 

 ning of the seventeenth century Van Helmont had put forward 

 the view that plants manufacture the greater portion of their 

 substance from water alone. He had found that a willow twig 

 planted in a weighed pot of earth had increased in weight by 

 161 lb. after five years, although nothing but water had been 

 supplied and the dry weight of the soil had only decreased by 

 two ounces. However as chemistry progressed such an idea 

 became altogether untenable. 



Towards the end of the same century Mariotte, better known 

 for his discovery of laws relating to gases, applied chemical 

 notions to plants. The various incombustible substances remain- 

 ing after plants were distilled were held by him to have been 

 formed by the combining together in various proportions of 

 certain simpler substances absorbed from the soil. Hence 

 Mariotte credited the plant with ability to change the absorbed 

 materials into others. Still this did not explain the fact that 

 plants derive only a small fraction of their total substance from 

 the soil, as shown by Van Helmont's experiments. It remained 

 for Stephen Hales, in 1727, to suggest in his Statical Essays that 

 air too was a constituent of vegetable matter. He observed that 

 air could enter through the leaves and through certain openings 

 in the rind ; connecting its presence in living plants with the 

 evolution of " air " on distilling them, he concluded that the 

 atmosphere actually entered into the composition of the plant. 

 For half a century after Stephen Hales' work appeared no 

 real advance was made. But the discovery of oxygen by 

 Priestley in 1774, and of the composition of " fixed air" (carbon 

 dioxide) and of water by Lavoisier, 1776-83, served to prepare 

 the ground for further investigations into the functions of leaves. 

 In 1779 Priestley found that oxygen was often exhaled by the 



