26 SCIENTIFIC AND APPLIED PHARMACOGNOSY 



The action of yeasts upon sugars is to form alcohol and carbon 

 dioxide, the former being the important product in the brewing 

 industries and the latter for the raising of the dough in bread making. 

 This reaction is due to the production of the enzyme zymase. Yeast 

 also contains the enzyme known as invertase. In commercial mix- 

 tures other enzymes are doubtless present.. The use of yeast in 

 medicine is due to the collective action of the several enzymes and 

 for this purpose both brewer's yeast and compressed yeast are, 

 employed. The latter is used under the name of Faex Compressa 

 and contains besides yeast, starch grains and a few bacteria. Com- 

 pressed yeast has a characteristic odor and should be free from molds 

 and unpleasant odors. 



LICHENS 



Lichens are a peculiar group of plants being composed of some of 

 the higher fungi which are parasitic upon some of the green and blue 

 Algae. They are of rather common occurrence upon the barks of 

 trees, rocks and some grow upon soil. They consist of a thallus in 

 which the algal cells have a more or less definite position. The upper 

 portion consists of compact hyphas comprising what is known as the 

 cortical layer. Beneath this is the algal layer from which the fungal 

 hyphse draw their nourishment. The lower surface is differentiated 

 into a cortical layer from which the hyphae are extended in the form 

 of rhizoids attached to their substratum. The fruit or sporocarp 

 varies, depending upon the fungus entering into the lichen, but the 

 apothecium is the most common. Owing to the fact that many 

 of them contain lichenin, a carbohydrate resembling starch, a few of 

 the lichens have been used as food. While some are of medicinal 

 interest, as Cetraria, their chief interest is in the coloring principles 

 which they contain and which have been the subject of painstaking 

 investigations of O. Hesse during a great many years. 



CETRARIA. Iceland Moss. The entire dried plant of Cetraria 

 islandica, one of the Ascolichens which is widely distributed over the 

 northern part of both continents. The chief commercial supplies 

 are obtained from Scandinavia, Germany, Switzerland and parts of 

 Austria. 



Description. Consisting of a number of somewhat dichotom- 

 ously branching, more or less curled, papery, fringed segments, 

 5 to 10 cm. long and about 5 mm. wide; upper surface greenish- 

 brown, with occasional dark reddish-brown cupular apothecia; 

 under surface grayish, with numerous small, whitish, depressed 



