378 THE UNIVEKSE. 



minatecl by a little swelling or spongiole, to which the 

 function of absorbing is specially intrusted, and which, 



204. Spoiigiole of the Pontederia. 



like an invisible sponge, absorbs the nutrient juices of the 

 soil which surrounds it. 



CHAPTER II. 



THE STEM. 



Although the diversified forms of stems do not allow 

 us to classify them strictly, Ave can at least see that they 

 often present themselves under three strictly defined as- 

 pects, the types of which arc found in our trees, in the 

 palms, and in the grasses. 



The stem of our trees — called the trunk, consists of a 

 greatly cLjugated cone, which l)ecomes very much smaller 

 as it gains in height. In a section of it vfe distinguish 

 three parts clearly defined — the bark, the wood, and the 

 pith. 



The bark, which is the outermost, is foi^mcd of toler- 

 ably numerous layers. The parts most deserving of re- 

 mark in it are the epidermis, a fine transparent membrane, 

 A\diicli generally allows us to see the tissue over which 



