70 PRACTICAL BOTANY 



limit of the bundle. The walls are thick and lignified 

 (yellow with chlor-zinc-iodine, or with acidulated 

 aniline sulphate, see p. 37) ; they have no protoplasmic 

 contents their further distinctive characters can only 

 be seen in longitudinal sections. Thyloses may be 

 observed (see below, p. 75), especially in more central 

 vessels. The vessels are embedded in a mass of tissue 

 composed of two tissue-forms, which, however, are not 

 readily distinguishable in transverse sections : they are 



b. Xylem-, or wood-fibres, which appear irregular 

 and polygonal in transverse section, and have thick 

 lignified walls : cell-contents are not prominent, or they 

 may be entirely absent. 



c, Xylem-parenchyma cells which retain their 

 protoplasmic contents ; their cell- walls are lignified, or 

 of cellulose : the latter is the case with those cells which 

 surround the more central vessels. This constituent 

 of the bundle is more characteristically represented in 

 the stem of the Elm (see below, p. 96). 



5. The pith consists of cells, which have for the 

 most part lost their cell-contents : they have very thin 

 walls ; the walls are slightly pitted : intercellular spaces 

 small. The cell-cavity is usually filled with air, which 

 replaces the protoplasm ; this is especially the case near 

 the centre, hence the whiteness of the pith. 



III. Cut radial longitudinal sections of an old stem 

 of Helianthus, and choosing such as have passed through 

 a vascular bundle (easily recognized with the naked 

 eye), treat them as above. 



Bear in mind the observations already made on the 

 transverse sections, and compare those results with the 

 observations about to be made. 



