THE VEGETABLE KINGDOM. 349 



On the contrary, the water-lentil, which spreads its car- 

 pet of verdure on the surface of our pools, possesses nothing 

 but spongioles. A Pontederia, which floats on the rivers of 



100. Spongiole of the Floating Pontederia [Pontederia crassipes), highly magnified. 



India, is only furnished with fine rootlets spreading through 

 the water. But these are rare exceptions. 



Buried in the earth, the root there performs its three 

 functions in obscurity. For this purpose each of its capil- 

 lary filaments is terminated by a little swelling called spon- 

 giole, to which the function of absorbing is specially in- 

 trusted, and which, like an invisible sponge, sucks up 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, we can at least see that they often 

 present themselves under three strictly defined aspects, the 

 types of which are found in our trees, in the palms, and in 

 the grasses. 



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

 greatly elongated cone, which becomes very much smaller 

 as it gains in height. In a section of it we distinguish three 

 parts clearly defined, the bark, the wood, and the pith 



