200 VEGETABLE ORGANOGRAPHY. 



tumified knots gives a singular appearance to the sub- 

 terranean part of the stem. A variety of the Wild Oat 

 (Avena elatior), which has been called A. precatoria, 

 or Necklace-like Oat, presents this structure. The knots 

 situated above the earth are never swollen in a remark- 

 able manner except by accident. 



The internodes are covered in the lower part, or 

 throughout the whole of their length, by the sheaths of 

 the leaves : this covered part is always softer and more 

 herbaceous than the part exposed to the air ; it never 

 presents any hairs, and rarely stomata so well developed 

 as those of the exposed part. 



The interior and central part of the whole length of 

 the internode is always softer than the exterior, and 

 only presents a dilated cellular tissue, which, in its 

 young state, is filled with watery juice, drying up as it 

 grows older ; sometimes it remains entire ; at other 

 times it is more or less completely broken, forming the 

 hollow Culms, or those cavities in reeds which run from 

 one knot to another. 



All that we have said of the Graminese is applicable 

 to the Calami, which belong to the family of the Palms, 

 but have their stems marked here and there with knots 

 which bear the leaves and give birth to axillary buds. 



§ 5. — Of the Stem of the Equisetums. 



The stems of the Equisetums have much analogy with 

 those of the Gramineee, but they seem, at first sight, 

 to approach a little more to those of Exogens ; they are 

 cylindrical, furnished with solid knots, from which 

 spring the verticels of branches and leaves. The inter- 

 node presents, in its interior, a central cellular tissue, 

 which is very early broken, so as to form a hollow cylin- 



