THE ROMANCE OF OUR TREES 



mere framework of cell-chambers containing merely 

 waste products such as crystals of calcium-oxalate 

 which are thrown off with the leaves and help to en- 

 rich the soil. But while the process of food evacu- 

 ation is going on other changes take place. In many 

 plants a chemical substance, known technically as 

 anthocyanin, is produced in the leaves and often 

 to such an extent as to become plainly visible on the 

 exterior. It appears red in the presence of free acids 

 in the cell-sap, blue when no acids are present, and 

 violet when the quantity of acids is small. In a 

 great many leaves the bodies which contain the 

 green colouring matter become changed to yellow 

 granules while the evacuation of food substances is in 

 process. Sometimes these granules are very few 

 and anthocyanin is absent, then the leaf exhibits 

 little outward change except losing its freshness be- 

 fore it falls. In others the yellow' granules are 

 abundantly developed, and if anthocyanin is absent 

 or nearly so the whole leaf assumes a clear yellow 

 hue. If there is an abundance of yellow granules 

 together with free acids and anthocyanin the leaf 

 assumes an orange colour. Thus the leaf at the 

 period of autumnal change by the presence of these 

 substances in a greater or lesser degree loses its green 

 hue and becomes brown or yellow, crimson or or- 

 ange, purple or red. The play of colour is greater 



38 



