264 ESSENTIALS OF BOTANY 
work of photosynthesis. Fungi are therefore destroyers of 
food, and (like animals) on the whole lessen the total amount 
of organic matter on the earth, while green plants tend to 
increase it. In fungi, formation of fat or oil, especially as 
reserve material, takes the place of the starch making so 
common in plants with chlorophyll. 
As regards their mode of life, fungi may be roughly di- 
vided into three groups: (1) pure saprophytes; (2) plants 
which are saprophytic or parasitic according to circum- 
stances ; (8) plants which can only reach their full de- 
velopment as parasites. Several of the classes of fungi 
mentioned in Sect. 805 contain representatives of all three 
of the groups given above. The common mushroom is 
a familiar example of the first group, Armillaria mellea 
(Fig. 185) is a member of the second group, and the rusts 
of Figs. 181-183 belong to the third group. 
