26 GUIDE TO THE STUDY OF LICHENS. 



If we scrape off a small fragment of the green coat- 

 ing of a tree or flower-pot and examine it under a 

 microscope we find that it consists of numerous minute 

 globules, perhaps five-thousandths of an inch in diam- 

 eter. Each globule is an individual alga and is nothing 

 more or less than a single cell. Not all of the lower 

 algae are single-celled however ; many consist of chains 

 or filaments of cells or even of a highly complex struct- 

 ure. The algae with which we are concerned are either 

 single-celled or form simple or branching chains of 

 cells. Their structure will be more fully described 

 later. 



Algae contain a green substance known as chloro- 

 phyll. It is this substance which enables the organism 

 to assimilate inorganic food ; a function peculiar to all 

 chlorophyll-bearing plants and which distinguishes them 

 from animals and parasitic plants. Animals and fungi 

 can only assimilate the food-substances directly or in- 

 directly prepared by the chlorophyll-bearing plants. 

 This brief reference to the algae will suffice for the 

 present. The facts to be distinctly kept in mind are 

 that alg£e contain chlorophyll and have the power of 

 assimilating inorganic food-substances under the influ- 

 ence of sunlight. Now as to the fungi. 



Every one has seen toadstool-s, puff-balls, mould 

 on bread and other organic substances kept in moist 

 and rather dark places, rust on wheat and other cereals, 

 smut on corn, etc.; all of these are fungi. Every one 

 has heard of bacteria and is more or less familiar with 

 the role they play in disease ; some have no doubt 

 heard of the importance of bacteria in the manufacture 



