SUNFLOWER STEM 41 



elements of the mature xylem and phloem from the originally 

 uniform cells of the cambium. For further details see the 

 Elm (pp. 56, 63) which, being a woody stem, and having more 

 definite secondary increase, is a better type for the study of 

 cambium. (Compare Fig. 7, A, p. 64.) 



iv. The xylem also consists of elements of various structure : 

 of these the most noticeable are 



a. The vessels, easily recognized by their large cavity : they 

 are arranged in radial rows, the individual vessels usually 

 decreasing in size towards the central limit of the bundle. 

 The walls are thick and lignified (yellow with chlor-zinc-iodine, 

 or with acidulated aniline sulphate, see p. 25) ; they have no 

 protoplasmic contents, their further distinctive characters can 

 only be seen in longitudinal sections. Thyloses may be ob- 

 served (see below, p. 44), 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. 56). 



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 ; 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 



