These substances, after undergoing certain changes in the vessels of the plant, are exposed to the influence of the rays of the sun in the leaves.

By the power of these rays--including the calorific, the luminous, and the actinic--the natural affinities by which the above-mentioned substances were united are overcome, and they are formed into new combinations, in which they are united by very weak affinities.

Of course, they have a strong tendency to break away from the new unions, and fall back into the old.

But, by some mysterious and incomprehensible means, the sun has power to lock them, so to speak, in their new forms, so as to require a special condition of things for the releasing of them.

Thus they form a reserve of force, which can be held in restraint until the conditions required for their release are realized.
The process can be illustrated more particularly by a single case.