280 GENERAL BOTANY 



The relation of the fungus to the algae is therefore that of a 

 parasite on a green plant, while the algae probably derive some 

 benefit from the fungus in raw food materials and protection. 

 Such a relationship is called symbiosis, which means a partnership 

 with mutual advantage to each partner. This physical relation 

 of the algae and the fungus of the lichen body is easily demon- 

 strated by making sections or by teasing out small portions of 

 an ordinary lichen in a drop of water. The striking contrast 

 between the bright-green algal cells and the colorless hyphae of 

 the fungus enables one to determine without difficulty in such 

 preparations the relation and the nature of the two plant partners. 



Reproduction. Almost any part of the plant body of a lichen, 

 if removed from the mother plant and placed under favorable 

 conditions, can reproduce the plant vegetatively. The usual 

 method of reproduction, however, is by means of asexual spores, 

 which are formed in special club-shaped cells termed asci. These 

 spore-bearing cells occur in great numbers in the so-called lichen 

 cups, which are easily seen covering the upper surface of lichens 

 in fruit. The inner surface of these cups, or apothecia, is usually 

 dark brown or gray in color, but in some instances it may be 

 orange or brick red. A section through the cup (Fig. 156, #) 

 shows that its entire inner surface is lined with the club-shaped 

 spore-bearing cells, or asci, alternating with greatly elongated 

 sterile hyphae called paraphyses. Each club-shaped spore-bearing 

 cell contains from two to eight spores, which are expelled with 

 some force from the ascus when they are ripe. These, spores ger- 

 minate like mold spores, and if the hyphae thus formed come 

 in contact with the proper algae, they attach themselves to the 

 algal cells and gradually form a new lichen plant. It is thus 

 seen that the production of a new lichen is entirely dependent 

 upon spore reproduction by the fungus. In this manner lichens 

 arise in nature wherever rough, moist surfaces offer the proper 

 conditions for their growth. On account of their great hardiness 

 and their ability to make their own food the lichens are among 

 the most widely distributed plants in nature. 



