ORGANIZATION OF THE STEM. 61 



The ligneous zones, considered collectively, are more indu- 

 rated towards the interior of the stem, because, in fact, these 

 are the most ancient deposits ; but, examined separately, they 

 are more solid in their exterior than in their interior parts; 

 because the latter was formed in early spring, at a period when 

 the nutritive sap was more aqueous and less condensed. 



The Medullary Sheath. This is a very thin zone of vascu- 

 lar tissue and spiral fibre, immediately surrounding the pith, 

 shown at dd. It may be readily seen in the traverse section 

 of a young exogenous shoot by its green color, which appears 

 deeper as contrasted with the white of the pith which it sur- 

 rounds. If we scoop out the pith of the shoot from the 

 ligneous cylinder which surrounds it, we shall obtain a longi- 

 tudinal view of the medullary sheath, which will appear like 

 a green layer on the interior surface of the cylinder. The 

 medullary sheath is the earliest formed portion of the vascular 

 system, and is developed with the upward elongation of the 

 stem, sending its woody fibre and spirals into each young shoot 

 and leaf to form its veins. The medullary sheath is the only 

 part of an exogenous stem in which spiral vessels occur. 



The Pithy represented at a, consists of soft cellular tissue, 

 and is formed as the stem elongates. At first it abounds with 

 nutritive matter, which serves to nourish the growing bud 

 resting on its summit ; this office fulfilled, it becomes dry and 

 dies, assuming the appearance and structure of wood, insomuch 

 that in old stems, there is scarcely such a thing as pith to be 

 seen. Herbs and young shrubs, in proportion to their bulk, 

 have more pith than trees. Many herbaceous stems expand so 

 rapidly during their early growth that they become hollow, 

 the pith being torn away by the distension and its remains 



6* 



