NUTRITION. 35 



nous trees the Pith, or Medulla, in. the centre; 

 the Wood surrounding the pith ; the Bark which 

 envelopes the whole, and the Medullary Hays, 

 which run horizontally across the wood and 

 bark, from the centre to the circumference. To 

 these may be added the Medullary Sheath which 

 is but the first annual layer of wood. 



27. The Pith, or Medulla, is composed of 

 cellular tissue, whose cells are large, regular, 

 and spongy, it contains starch which is after- 

 wards converted into mucilage,* and its office 

 seems to be that of nourishing the young buds ; 

 when this function is performed it perishes. 

 Around it is the Medullary Sheath which differs 

 from the succeeding annual layers only in 

 having its vessels usually capable of being un- 

 rolled, and consequently truly spiral, it enve- 

 lopes the pith like a case, and its fibres often 

 branch into the substance of the pith itself, 

 where they appear as scattered spiral vessels. 

 The medullary sheath has been supposed to be 

 the channel by which oxygen, liberated by the 

 decomposition of carbonic acid, is conveyed to 

 the leaves. 



* See " Introduction to Practical Organic Chemistry," 

 P- 49, $ 36. 



