THE STEM OF VASCULAR PLANTS. 201 



der, composed of two rows of fibres, an external and an 

 internal one, disposed so as to alternate with each other : 

 seen through the microscope, they are composed of striped 

 vessels intermixed with dotted ones, and elongated cel- 

 lules ; the external ring presents tubular air-cavities, 

 disposed with great regularity. 



In the knots the central cellular tissue is not broken, 

 and seems to enjoy the function of pith : from the ex- 

 ternal border of it, or from the internal part of the 

 external cylmder, there proceed striped vessels, in a 

 horizontal direction, which go to the surface, and are 

 there developed into branches which are organized as 

 the stern. 



All the parts of the Equisetums which we have been 

 accustomed to call stems, are annual, and spring from a 

 rhizoma, or subterranean stock, the organization of 

 which deserves a very attentive examination, because it 

 appears capable of attaining an extremely advanced age, 

 and might, consequently, give us some idea of the mode 

 of growth of Endogens. 



§ 6. — Of the Stem of Ferns. 



The stem of Ferns presents itself under three very 

 distinct appearances : sometimes it is erect, firm, cylin- 

 drical, and simple, as that of Palms ; this we see in 

 Cyathea spinosa, Dicksonia, &c. : sometimes it is weak, 

 climbing, twining around trees, branching, but mani- 

 festly cyhndrical in each ramification — as, for example, 

 in Ugena, &c. There are some which have it creeping 

 upon the ground, as in Polypodium Virginicum. Finally, 

 in the small Ferns peculiar to our chmates, the stem creeps 

 along under the surface of the ground instead of above 



