66 COMPOUND ORGANS OF PLANTS. 



last leaves of the season, developed at a period when the sap 

 is ceasing to flow, and when the vital powers of plants have 

 become almost torpid. The transition of scales into the 

 ordinary leaves of the stem is well seen in the spring, in the 

 expanding buds of the hickory or horse-chestnut, where the 

 gradual passage of one into the other may be distinctly 

 traced. 



Buds originate in the horizontal or cellular system, and may 

 be distinctly traced in young branches to the pith or medullary 

 rays, at the extremities of which they are invariably found 

 when they take a lateral development. This may be easily 

 verified by making a section through the centre of one of the 

 lateral buds, at right angles to the surface of the stem, when 

 the medullary ray will be seen on the surface of the section in 

 the form of a white line, which, proceeding from the centre of 

 the bud, traverses the several rings or annual deposits of wood, 

 and terminates in the pith at the centre of the stem. The 

 central cellular portion of every bud is therefore in direct 

 communication with the interior pith of the young shoot by 

 means of the medullary rays, at the extremities of which they 

 are formed. 



Buds are formed some in the early part of the summer, 

 others late in autumn, before the leaves fall from the trees in 

 the axilla of the leaves, that is, in the angle formed by the leaf- 

 stalk and the stem. Examine the branch of any tree before it 

 has cast its leaves, and you will find at the base of the petiole 

 or leaf-stalk, the buds for the ensuing year. Hence in 

 autumn, after the leaves have fallen, these buds remain 

 attached to the branches. 



Linnaeus called buds the hybernaculum or winter residence 

 of the branch ; and the term is very appropriate, because it 



