Important changes are now taking place in these perennial members of the vegetable world in preparation for the coming winter. The temperature of the soil is becoming considerably reduced, and, as a result, the absorbing activity of the roots is greatly decreased, while the winter is coming, when the temperature will be so low at times that the circulation of the sap will practically cease. If the leaves remained on the trees, they would give off from their surfaces more water than the trees could obtain from the soil through their inactive roots, thus endangering the lives of the trees. The leaves, therefore, must be shed. But these leaves contain a considerable amount of nutritious material which they themselves have built up, and which should not be lost. They contain starch, albumen, and other compounds which would be entirely lost to the trees if the leaves were shed in their present condition, except that a small proportion, in the form of products of decomposition, might be re-absorbed. This being the case, arrangements must be made, first, for the passage of the nutritious material in the leaves to some other part of the tree where it can be stored for the winter; and, second, for the removal of the leaves as the roots become less active. So, before the time of leaf-fall, the nutritious substances in the leaves, including the _chlorophyll_ to which the leaf owes its green colour, become changed, and pass back to the stems or the root, where they can be safely stored for the winter. The leaves, thus impoverished, become mere skeletons--mere collections of empty, lifeless cells; and if no further change takes place, they assume a very pale colour, like the leaves of the Hornbeam, Birch, and the Willows. But the transfer of the nutrient matter from leaf to stem or root is accompanied by numerous chemical changes by which new compounds are formed. Among these new substances a dark blue compound called anthocyanin is produced in some plants; and where this exists in considerable quantities we find the leaves of a dark bluish-green colour, like that of the autumn foliage of the Pine. Acids are also sometimes formed as a result of the complicated chemical changes that take place during the transfer above described; and these react on the anthocyanin present, changing its colour to a tint that varies according to the proportion and quantity in which they exist.