54 INTRODUCTION TO GENERAL PftYStOLOGY 



have seen that the simplest compound of the same percentage 

 composition as glucose is formaldehyde, CH.,0, and it is natural 

 to imagine that this might be the first result of what is sometimes 

 called " photo-assimilation," the taking up of carbon by the agency 

 of light. Although there is much probability that this is the case, 

 from the general properties of formaldehyde and other evidence, it 

 has not yet been found possible to obtain really satisfactory proof 

 of its production in the green leaf. This may be due to the fact 

 that it is a powerful chemical agent, and injurious to living 

 protoplasm if present in any but minute quantities. For this 

 reason it would be rapidly converted into sugar. In the laboratory 

 it can easily be caused to- unite, six molecules at a time, to form a 

 hexose. A further ground for the belief that formaldehyde is 

 formed by the action of light in the chlorophyll system is that 

 certain artificial systems, under the action of light, can produce it 

 from carbon dioxide and water. One of these is colloidal ferric 

 hydroxide under the action of ultra-violet light. This fact is of 

 further interest, because it suggests a possibility with regard to the 

 mechanism in the plant. To understand this we must consider 

 the form in which chlorophyll is present in the cell. It is insoluble 

 in water, and is therefore present in particles or colloidal solution, 

 and is not distributed generally throughout the cell substance, but 

 located in special structures usually, but not always, of spherical 

 shape (E., p. 1 86). These " chloroplasts " contain other things in 

 addition to chlorophyll, and of some of these it is interesting to 

 find that iron is a component. Although iron is very common 

 in cells, and has functions connected with oxidation, its presence 

 in the chloroplast is significant, and suggests that the function of 

 the chlorophyll itself may be to absorb light energy, bringing it 

 into intimate relationship with the chemical system, and that, after 

 absorption, the iron may come into play and cause the production 

 of formaldehyde, as in the ferric hydroxide above. 



At the same time, chlorophyll itself has so peculiar a chemical 

 structure that it is difficult to believe that this does not, in some 

 way, play a part. The suggestion has been made that carbon 

 dioxide and water are taken into combination with the pigment, 

 then reduced to formaldehyde, which is given off. But no evidence 

 exists that chlorophyll takes up carbon dioxide. By itself, even in 

 the presence of light and of carbon dioxide, it does not produce 

 formaldehyde or sugar. It appears that the other parts of the 

 chloroplast are necessary. 



Although the chemical nature of chlorophyll has not, as yet, 

 thrown much light on the photo -chemical reaction with which we 

 are concerned, this structure is in itself an interesting one. In 

 connection with the possible function of iron, it is remarkable that, 



