Re: Why can a photo negative be printed onto a plant's leaf after photosynthesi

It is an interesting experiment based in the fact that plants require light to make sugars.

When you cover a leaf with a film negative, different parts of the leaf will receive different amounts of light, since the negative has a "pattern of transparency" (some areas are more transparent than others). Taken together with the fact that plants use light as a source of energy to make sugar (glucose) in a process called photosynthesis (which means synthesis in the presence of light), it is easy to see that the negative leaves a pattern of photosynthesis "printed" on the leaf. Lighter areas make more photosynthesis and, therefore, more sugars.

The interesting comes now. If the leaf makes more sugar than it is able to use (or export to the rest of the plant), it stores the excess in the form of starch. In areas that receive more light, the cells will accumulate starch, while dark areas will not store starch.

When you wash the leaf with alcohol, you kill the leaf leaving it unable to degrade and export the starch accumulated. That means that you "freeze" the leaf with a pattern of starch that corresponds to the pattern of light incidence (that was defined by the negative). The alcohol is also used to remove the pigments that would increase the background of your "photo" and reduce the contrast, specially chlorophyll.

The last step is to use iodine to stain the starch. The ion formed in the iodine solution interacts specifically with the starch turning it black.
Voila!! There is your photo printed on the leaf.
Despite the apparent simplicity of the experiment, it involves quite advanced concepts in plant physiology.