SUNFLOWER. STEM. 57 



infra, p. 61], 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 not prominent, or 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 con- 

 stituent of the bundle is often absent, and is not 

 characteristically represented in this case (cf. stem of 

 Elm, infra). 



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

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

 walled; the walls are slightly pitted: intercellular spaces 

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

 replaces the protoplasm, especially 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 recognised with the naked 

 eye), trea.t 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. 



To complete the study of the tissues it would be necessary also 

 to cut tangential sections, and, in the case of tissues in which the 

 radial differ from the tangential walls, such sections must be made, 

 and the comparison drawn between them and the transverse and 

 radial sections (cf. stem of Pinus). In the present case, however, 



