24 



PITH. WOOD. 



34. In herbaceous plants, and in ligneous plants of very 

 rapid growth (such as the elder), the space occupied by the pith 

 is very considerable ; but in trees, the wood of which is very 

 hard, such as the oak, the medullary canal or sheath is generally 

 very small. 



35. The parietes of the canal, containing the central pith, 

 called the medullary sheath, are formed of longitudinal fibres, 

 ordinarily arranged in a circle, and of a layer composed of tra- 

 cheae, false tracheae, and porous vessels. It is the only part of 

 the stem in which true tracheae have been observed. 



36. Between the medullary canal and the bark, is the ligneous 

 body, or wood, which is composed of concentric layers, the 

 number of which is more or less considerable, according to the 



c b a 



Fig. 13. TRANSVERSE SECTION OF AN Ex'OGENOUS STEM. 



age of the plant (Jig. 13, 6, c) ; each of these layers is com 

 posed of longitudinal fibres, united to the subjacent layer by eel 

 lular tissue. These fibres are formed nearly in the same manner 

 as those of the medullary sheath, except that no tracheae are 

 found in them ; they are composed only of clostres or elongated 

 cells, or dotted or rayed vessels. 



37. The ligneous body constitutes what is generally termed 

 wood ; its central portion is harder than its external part, and is 



Explanation of Fig. 13. Transverse section of an ex'ogenous stem : 

 a. the pith ; b. layers of the heart of the wood ; c. layers of the albur'num 

 or sap-wood ; d. the bark. 



34. How does the pith vary in quantity in different plants ? 



35. What is meant by medullary sheath ? What is remarkable in its 

 structure ? 



36. What is meant by the ligneous body ? How is it formed ? 



37. What is wood ? What is meant by true wood ? What is meant by 

 alburnum ? In what respect does true differ from sap wood ? 



