This was developed further into Watt's "Dictionary of the Economic Products of India" published from 1889 to 1893 and projected to be in six volumes but it grew larger and the additions were accommodated by breaking up the last volume into four parts. It is considered by many as the greatest compilation of the commercial plants (and natural products) of India. Although there were similar subsequent works like the "Wealth of India" series, Watt's work continues to be a major source for research. Watt's compilation included descriptions of other natural products such as silk, oysters and minerals.