Algas, Fungi, and Lichens. 279 



and finally produce enlargements at their ends which 

 become separated from the rest of the branch by a cell 

 wall (Fig. 140). The protoplasm in these enlarged ends 

 divides into many spores, which finally break out from the 

 surrounding wall and become scattered, giving rise on their 

 germination to new filamentous individuals. 



When the substratum supplies an abundance of food 

 materials, resting spores may be produced sexually. In 

 this process the ends of two branches meet, and a partition 

 wall is formed in each, a short distance back from the 

 ends. The end walls then become dissolved, and the con- 

 tents of the short terminal cells fuse together, thus consti- 

 tuting a single cell, which then enlarges considerably, 

 produces a thick outer wall, and enters into a period of 

 rest. When this sexually produced spore germinates, the 

 thick outer wall becomes broken, and the inner wall and 

 contents grow forth and produce a branched filament 

 which sooner or later gives rise to asexual spores as above 

 described (see Fig. 140). By the asexual spores, which 

 are produced in almost countless numbers, the fungus be- 

 comes broadly scattered, while the less number of sexual 

 spores serves to carry the species through adverse condi- 

 tions. 



173. Rusts. — The rusts on wheat and other grasses are 

 interesting representatives of a large class of Fungi which 

 obtain their living at the expense of other live plants. 

 Observations 163 and 164 have shown u" ti.at the rusty 

 streaks appearing on the leaves of wheat, for instance, 

 are really masses of the c.al spores of the fungus (Fig. 

 141, A) which break through the epidermis and become 

 blown about by the wind. The spores are borne on the 

 filamentous vegetative part of the Fungus (Fig. 141, B), 

 which ramifies amongst the parenchyma cells of the leaf 



